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THE   CORAL  SIDERASTREA   RADIANS  AND 
ITS   POSTLARVAL   DEVELOPMENT. 


BY 


J.  E.  DUBRDBN. 


Washington,  U.  S.  A. 

Published  by  the  Carnegie  Institution, 

December,   1904. 


THE   CORAL  SIDERASTREA    RADIANS   AND 
ITS   POSTLARVAL   DEVELOPMENT, 


BY 


J.  B.  DUBRDBN. 


Washington,  U.  S.  A. 

Published  by  the  Carnegie  Institution, 

December,   1904. 


Carnegie  Institution  of  Washington, 
Publication  No.  20. 


Press  of  Gibson  Bros., 
Washington,  D.  C. 


CONTENTS. 


Preface "^ 

Introduction i 

Adult  Colony. 

External  Characters 7 

Column  wall 8 

Tentacles 9 

Disc  and  mouth 14 

Color 15 

Reproduction 16 

External  characters  on  decalcification 18 

Anatomy  and  Histology 21 

Column  wall  and  disc 3i 

Tentacles 33 

Stomodaeum 23 

Mesenteries 24 

Mesenterial  filaments 29 

Skeletotrophic  tissues 30 

Septal    invaginations,    interseptal    loculi, 

and  gastro-coelomic  cavity 34 

Gonads 37 

Corallum 38 

Histology 40 

Wall  or  theca 45 

Septa 46 

Synapticula 52 

Columella 53 

Dissepiments 55 

Epitheca  and  basal  plate 56 


POSTLARVAL   DErKLOFMKNT. 

Larva 57 

Young  polyp 61 

Tentacles 65 

First  cycle  of  exotentacles 65 

First  cycle  of  entotentacles 66 

Secondary  exotentacles 69 

Second  cycle  of  entotentacles  and    third 

cycle  of  exotentacles 71 

Third  cycle  of  entotentacles   and    fourth 

cycle  of  exotentacles 73 

Mesenteries 76 

First  cycle  of  mesenteries  (protocnemes)...  76 

Second  cycle  of  mesenteries  (metacnemes)  79 

Third  cycle  of  mesenteries 83 

Corallum 86 

First  cycle  of  entosepta  and  second  cycle 

of  exosepta 86 

Second  cycle  of  entosepta  and  third  cycle 

of  exosepta 93 

Third  cycle  of  entosepta  and  fourth  cycle 

of  exosepta 99 

Basal  plate II3 

Epitheca iij 

Columella 117 

Anatomy  and  histology  of  larva  and  young 

polyp 119 

Young  polyps 122 

References 125 

Explanation  of  plates 126 

iii 


PREFACE. 

The  researches  of  the  late  Prof.  H.  de  Lacaze-Duthiers  (1873,  1897), 
Prof.  G.  von  Koch  (1882,  1897),  and  Prof.  H.  V.  Wilson  (1888)  have  made 
us  acquainted  with  many  of  the  early  stages  in  the  development  of  corals. 
They  have  served  to  establish  such  fundamental  facts  as  the  ectodermal 
origin  of  the  madreporarian  skeleton  and  the  sequence  of  the  primary  mesen- 
teries and  septa,  results  which  must  ever  possess  an  importance  to  the  student 
of  the  Anthozoa.  But  for  an  understanding  of  many  of  the  problems  of  adult 
coral  morpholog}^,  especially  those  associated  with  the  relationships  of  the 
mesenteries  and  septa,  it  has  long  been  desirable  that  developmental  stages 
later  than  those  studied  by  the  authors  mentioned  should  be  investigated. 
While  resident  in  the  West  Indies  I  have  followed  day  by  day  the  postlarval 
growth  of  the  coral  Siderastrea  radians  (Pallas)  for  a  third  of  a  year,  and 
secured  the  development  of  the  tentacles  and  septa  as  far  as  the  third  cycle, 
and  that  of  the  mesenteries  to  the  completion  of  the  second  cycle.  The 
results  are  herein  set  forth. 

In  many  respects  the  mature  polyps  of  S.  radians  are  of  peculiar  mor- 
phological interest,  but  have  never  been  fully  described.  An  account  is 
therefore  first  given  of  the  external  characters  and  internal  anatomy  of  the 
adult  colony,  and  afterwards  of  the  development  of  the  young  polyp  from 
the  free-swimming  larva.  The  manner  of  appearance  and  the  relationships 
of  the  tentacles,  mesenteries,  and  septa  are  considered  at  some  length,  their 
establishment  being  the  principal  object  of  the  investigation. 

The  work  was  commenced  while  Curator  of  the  Museum  of  the  Institute 
of  Jamaica,  continued  as  Bruce  Fellow  at  the  Johns  Hopkins  University, 
Baltimore,  and  concluded  at  the  American  Museum  of  Natural  History,  New 
York.  For  facilities  afforded  in  carrying  out  the  investigations  I  am  under 
obligations  to  the  Board  of  Governors  of  the  Institute  of  Jamaica,  Prof.  W.  K. 
Brooks,  of  the  Johns  Hopkins  University,  and  Prof.  H.  C.  Bumpus,  of  the 
American  Museum  of  Natural  History.  The  research  has  been  assisted  by 
an  appropriation  from  the  Carnegie  Institution. 

J.  E.  D. 

University  of  Michigan,  Ann  Arbor,  Mich.,  U.  S.  A., 
iiTH  November,  1904. 


THE  CORAL  SIDERASTREA   RADIANS  AND   ITS  POSTLARVAL 

DEVELOPMENT. 


By  J.  E.  DUERDEN. 


INTRODUCTION. 


Tlie  following  are  the  more  important  references  to  and  synonyms  of  this 
well-known  species  of  coral : 

Madrefora  radians,  Pallas,  Elench.  Zooph.,  1766,  322. 

Madrepora  astroites,  Linnaeus,  Sjs.  Nat.,  ed.  xii,  1767,  1276. 

Madrefora  galaxea,  Ellis  &  Solander,  Nat.  Hist.  Zooph.,  1786,  168,  pi.  47,  fig.  7. 

Astrea  galaxea,  Lamarck,  Syst.  Anim.  s.  Vert.,  1801,  371;  Le  Sueur,  M^m.  Mus.  Hist.  Nat.  Paris,  t. 
VI,  1820,  285,  pi.  XVI,  fig.  13;  Lamouroux,  Expos.  Mdth.,  1821,  60,  pi.  XLix,  fig.  7. 

Astrcea  radians,  Oken.  Lehrb.  Naturgesch.,  1815,  bd.  i.  65;  Milne-Edwards  &  Haime,  Hist.  Nat.  Cor., 
1857,  t.   II,  506;    Gregory,  Quart.  Journ.  Geol.  Soc.  Lond.,  vol.  Li,  1895,  277. 

Astrea  {Sider astrea)  g-alaxea,  de  Blainville,  Diet.  Sci.  Nat.,  1830,  torn.  LX,  335;  Man.  Actin.,  1834, 
370. 

Astrcea  astroites,  Ehrenberg,  Corall.  roth  Meer,  Abhandl.  kongl,  Akad.  Wiss.  Berlin,  1832,  319. 

Siderina  galaxea  {furs),  Dana,  Zooph.  Wilkes  Expl.  Exped.,  1846,  218,  pi.  x,  figs.  12,  12*,  12c. 

Siderastrcea  galaxea,  Milne-Edwards  &  Haime,  Ann.  Sci.  Nat.,  1850,  torn,  xii,  139;  Pourtal^s,  Deep-Sea 
Corals,  111.  Cat.  Mus.  Comp.  Zool.,  no.  iv,  1871,81;  Florida  Reefs,  Mdm.  Mus.  Comp.  ZooL, 
1880,  vol.  VII,  pt.  I,  pi.  XI,  figs.  14-21,  pi.  XV,  figs.  1-12;  Quelch,  Reef  Corals,  Challenger  Reports, 
1886,  vol.  XVI,  113;  Heilprin,  Proc.  Acad.  Nat.  Sci.  Phila.,  1890,  305. 

Siderastral^e'la  radia7is,  Verrill,  Bull.  Mus.  Comp.  Zool.,  vol.  i,  1864,  55  ;  Vaughan,  Bull.  U.  S.  Fish  Com- 
mission 1900,  vol.  2,  1901,  309,  pi.  XV,  pi.  XVI,  fig.  2;  Samm.  der  Geol.  R. — Museums  in  Leiden, 
1901,  Ser.  II,  bd.  11,  61 ;  Verrill,  Trans.  Conn.  Acad.  Science,  vol.  xi,  1901,  153,  pi.  xxx,  fig.  i. 

The  generic  term  has  been  thoroughly  discussed  in  the  recent  papers  of 
Gregory  (1895,  p.  278),  Vaughan  (1900,  p.  154),  and  Verrill  (1901,  p.  88). 
Vaughan,  fortunately  for  coral  taxonomy,  shows  that  the  name  Astrcua^  first 
used  binomially  as  a  coral  genus  by  Lamarck  in  1801,  can  not  be  retained 
in  madreporarian  terminology,  having  been  employed  by  Bolten  in  1798  for 
a  group  of  gastropod  shells.  In  this  conclusion  he  is  supported  by  Verrill. 
The  next  generic  name  available  is  the  Siderasirea  of  de  Blainville,  1830. 
The  specific  term  radians  of  Pallas  (1766)  has  priority  of  the  galaxea  of 
Ellis  &  Solander  (1786),  and  Vaughan  holds  that  the  Madrepora  astroites 
of  the  twelfth  edition  of  Linnaeus  is  the  same  as  M.  radians  of  Pallas. 

The  species  is  very  common  throughout  the  West  Indies,  and  is  recorded 


2  SIDERASTREA    RADIANS. 

from  Vera  Cruz  and  Colon  on  tlie  mainland.  It  is  equally  plentiful  on  the 
Florida  Reefs  and  flats,  and  around  the  more  northern  Bermudas.  Gregory 
(1895,  p.  277)  records  it  as  fossil  from  the  Low-level  Reefs  of  Barbados  and 
the  Pleistocene  Reefs  of  the  Bahamas. 

A  glance  at  the  references  shows  that  Siderastrea  radians  has  been  fre- 
quently described  and  figured,  but  mainly  as  regards  the  corallum ;  Le  Sueur 
(1820,  p.  285),  Pourtales  (1871,  p.  81),  and  Verrill  (1901,  p.  153)  have,  in  addi- 
tion, contributed  brief  notes  upon  the  mode  of  occurrence  of  the  living  colonies 
and  the  characteristics  of  the  polyps. 

As  AstrcBa  radians  the  species  is  thus  systematically  described  by 
Milne-Bdwards  &  Haime  in  their  "  Histoire  Naturelle  des  Coralliaires " 
(vol.  II,  p.  506): 

Polypary  often  fixed  upon  the  Valuta  turbinellus  of  Linnaeus,  or  spherical  and  free. 
Budding  takes  place  at  the  point  of  union  of  several  calices.  Calices  subpolygonal,  appearing 
thickened  at  the  borders  as  a  result  of  the  strong  development  of  the  septal  system,  although 
the  walls  are  indicated  only  by  fine  lines.  Columella  formed  by  one  or  two  compact  tubercles, 
scarcely  visible,  more  distinct  in  young  individuals.  Three  cycles  of  septa  complete,  and,  in 
general,  a  variable  number  of  a  fourth  cycle,  unequal.  Interseptal  chambers  extremely 
narrow.  Septa  much  serrated,  strong,  very  regularly  crenulated  at  the  border,  nearly  equal, 
the  primaries  and  secondaries  a  little  larger.  The  teeth  serrated,  obtuse,  and  sub-equal. 
The  septa  of  the  second  cycle  are  fused  by  their  internal  border  to  those  of  the  preceding 
cycle.  When  the  septa  are  broken  from  above  they  are  found  to  be  united  by  strong  granules, 
the  spaces  between  the  granules  resembling  small  foramina.  A  specimen  in  this  condition 
has  been  considered  by  Lamarck  as  a  distinct  species  under  the  name  Astrcta  punctifera. 
In  a  vertical  section  the  columella  is  compact  and  strong,  the  septa  are  perfect  lamellae,  cov- 
ered with  a  radiating  series  of  large  granules  ;  the  dissepirhents  are  rudimentary,  horizontal, 
simple,  and  0.5  mm.  distant  from  one  another.  The  species  sometimes  forms  large  masses. 
The  large  diameter  of  the  calices  is  from  3  to  4  mm.,  their  depth  2  mm.,  or  a  little  more. 

An  excellent  engraving  of  a  colony  is  given  in  Bllis  &  Solander's 
"Natural  History  of  Zoophytes"  (1786,  plate  47,  fig.  7),  under  the  term 
Madrepora  galaxea.  In  his  report  on  '*  The  Stony  Corals  of  Porto  Rico,"  Mr. 
Vaughan  (1901)  adds  a  photographic  reproduction  of  a  flat  incrusting  colony 
and  also  one  showing  the  enlarged  calices  (plates  xv,  xvi).  In  the  plates 
accompanying  Prof.  Louis  Agassiz's  "Report  on  the  Florida  Reefs  "  (1880) 
are  reproduced  (plate  xv)  a  dozen  beautifully  executed  drawings  represent- 
ing the  polyps  and  skeleton  in  some  detail.  Many  of  the  actual  details  with 
regard  to  the  tentacles  and  septa  are  different  from  those  observed  on  the 
Jamaica  5.  radiajis^  but  there  seems  no  doubt  that  the  same  species  is 
intended  in  each  case.  The  drawings  of  the  polyps  are  interesting  as  show- 
ing for  the  first  time  the  bilobed  character  of  the  tentacles  and  their  irregular 
arrangement  at  varying  distances  from  the  center  of  the  disc. 


INTRODUCTION.  3 

The  species  occurs  plentifully  in  Kingston  Harbor,  Jamaica,  from  the 
shore  down  to  a  depth  of  5  or  6  feet.  Here  the  colonies  are  small,  and  sub- 
spheroidal  or  irregular  in  shape,  partly  or  wholly  incrusting  some  dead 
shell  or  stone.  Occasionally  nearly  globular  colonies  are  obtained  with  the 
polyps  equally  developed  all  over  the  surface.  Such  colonies  lie  exposed  on 
the  sea-floor,  or  partly  buried  in  beds  of  the  aquatic  phanerogam  Thalassia 
marina^  and  are  associated  with  other  free  corals,  such  as  Manicina  areolata 
(Linn.),  Cladocora  arbuscula  (Le  Sueur),  and  Porites  divaricata  (Le  Sueur) ; 
sometimes  they  are  found  incrusting  shells  inhabited  by  the  living  mollusc 
or  by  hermit  crabs.  A  flat  incrusting  form,  growing  upon  stones  and  blocks 
of  coral,  occurs  on  the  coral  flats  outside  the  harbor  in  the  area  of  reef 
formation.  Here  the  species  may  give  rise  to  large  fixed  masses,  50  or 
60  cm.  across,  while  in  shallow  water  the  free  colonies  rarely  exceed  a 
diameter  of  10  cm.  On  the  reefs  S.  radians  is  associated  with  another 
common  West  Indian  species  of  the  same  genus,  viz,  S.  siderea  (Bll.  &  Sol.). 
This  latter  may  produce  large  hemispheroidal  blocks,  and  in  situ  is  readily 
distinguished  from  S.  radians  by  the  larger  size  of  the  polyps  and  their 
reddish  brown  color. 

S.  radians  seems  peculiarly  hardy  as  regards  the  conditions  under  which 
it  will  thrive.  The  shore  waters  within  Kingston  Harbor  are  often  muddy 
from  the  action  of  the  strong  day  breezes,  and  at  times  the  living  polyps  are 
covered  by  deposits  of  sand  and  mud.  Around  Jamaica,  however,  the  colo- 
nies are  rarely  exposed  to  the  direct  rays  of  the  sun,  the  fall  of  the  tides 
being  very  restricted ;  but  further  north,  where  the  difference  between  high 
and  low  tides  is  greater,  both  Pourtales  and  Verrill  have  found  specimens 
so  situated  that  they  are  frequently  subjected  to  the  direct  rays  of  the  sun. 
With  regard  to  the  occurrence  of  S.  radians  in  Florida,  Pourtales,  in  "  Deep- 
Sea  Corals"  (1871,  p.  81),  writes: 

In  altitude  it  ranges  higher  than  any  other  coral  of  the  Floridian  fauna,  as  small  masses  are 
found  flourishing  in  pools  left  by  the  tide.  I  have  even  seen  small  clusters  left  partially  dry  in 
a  hot  sun,  keeping  up  a  communication  with  the  water  only  by  a  few  of  the  lowest  polyps  of 
the  group.  From  their  position  they  must  frequently  have  been  thus  exposed  without  incon- 
venience. According  to  Prof.  Agassiz  the  polyp  has  bilobed  short  tentacles  at  different 
distances  from  the  center. 

Prof.  A.  B.  Verrill  (1901,  p.  154)  gives  the  following  notes  as  to  the 
conditions  under  which  the  species  is  found  at  the  Bermudas  : 

This  species,  which  is  abundant  at  the  Bermudas,  is  more  hardy  than  most  reef  corals, 
for  it  can  live  and  grow  well  in  shallow  water  on  mud  flats,  where  it  is  laid  bare  by  nearly 
every  tide,  and  where  most  other  corals  would  be  smothered  in  the  mud,  though  ►S'.  siderea 


4  SIDERASTRKA    RADIANS. 

and  some  forms  of  Isophyllia  fragilis  are  usually  found  with  it  in  such  places.  It  is  often 
partly  buried  in  the  white  calcareous  mud  of  the  flats,  and  yet  seems  healthy  there.  It  is  also 
abundant  in  the  small,  shallow  pools  left  on  the  flats  by  the  tide.  But  it  is  equally  common 
on  the  reefs,  where  it  often  grows  larger.  It  is  also  found  well  grown  in  Harrington  Sound. 
Exposure  to  the  dry  air,  or  even  to  the  hot  sun,  for  an  hour  or  so,  does  not  kill  it,  if  it  be  wet 
beneath.     Probably  its  porosity  enables  it  to  absorb  sufficient  water  to  prevent  drying  up. 

The  natural  occurrence  of  the  living  coral  under  such  varying  condi- 
tions as  to  purity  of  water  and  exposure  indicates  that  this  species,  at  any 
rate,  is  not  so  extremely  sensitive  to  its  environment  as  corals  generally  are 
found  to  be. 

The  polyps  are  so  small  that  their  external  characters  can  be  fully  made 
out  only  with  the  aid  of  a  lens  or  a  low-power  microscope.  Further,  they 
differ  so  much  in  appearance,  according  to  the  amount  of  expansion  and 
retraction,  that  to  obtain  their  complete  characteristics  it  is  desirable  to  keep 
the  colonies  under  observation  for  some  time  and  subjected  to  various  condi- 
tions. As  the  stocks  often  lie  free  on  the  sea-floor,  they  can  be  collected 
without  any  injury  to  the  polyps,  and  are  sufficiently  small  to  be  kept  in  the 
laboratory  in  ordinary  glass  vessels.  With  a  little  attention  no  difficulty  is 
experienced  in  keeping  the  polyps  alive.  Indeed,  the  colonies  continued  to 
increase  in  size  during  confinement,  and  while  kept  under  observation  new 
polyps  began  to  arise  from  the  calices  of  previously  dead  areas. 

The  boring  bivalve  Lithodomus  appendiculatus  Phi.*  nearly  always 
occurs  within  the  corallum,  often  several  in  each  colony.  The  siphon  has 
a  dark  purple,  funnel-shaped  incurrent  aperture  and  a  small,  tube-like 
excurrent  aperture,  and  protrudes  a  short  distance  beyond  the  surface  of  the 
colony,  the  activity  of  the  mollusc  keeping  up  a  strong  circulation  of  water ; 
larvae  and  other  small  floating  objects  may  pass  in  rapidly  at  one  aperture 
of  the  siphon,  and  after  a  brief  interval  may  be  shot  out  from  the  other. 
Numbers  of  cirripedes  {Pyrgomd)  also  frequently  infest  the  corallum,  and 
assist  in  maintaining  a  current  of  water  over  the  living  colony.  It  seems 
not  unlikely  that  these  infesting  organisms  may  be  of  much  importance  in 
clearing  away  the  mud  and  sand  which  accumulate  on  the  surface  of  the 
polyps,  and  the  currents  produced  by  them  may  also  bring  food  within  reach. 

During  the  greater  part  of  the  day  the  colonies  in  the  laboratory  were 
kept  in  the  shade,  or  even  in  darkness,  by  placing  the  glass  receptacles 
under  some  cover.  Under  these  conditions  the  polyps  remained  partly 
expanded,  but  retracted  when  exposed  for  any  length  of  time  to  the  direct 
rays  of  the  sun.     In  the  early  morning  and  towards  evening  the  vessels  were 

•  For  the  determination  of  this  species  I  am  indebted  to  Prof.  L.  P.  Gratacap,  of  the  American  Museum 
of  Natural  History. 


INTRODUCTION.  5 

removed  for  a  short  time  into  the  full  rays  of  the  tropical  sun,  or  kept  for 
hours  in  diffuse  light,  when  all  the  polyps  expanded  to  their  fullest  degree, 
and  seemed  tingling  with  vitality  ;  but  in  general,  like  most  other  corals 
and  actinians,  the  polyps  seem  most  vigorous  in  the  shade  and  at  night  time. 
Detailed  experiments  on  the  phototropism  of  coral  polyps  are  much  needed. 

When  the  colonies  were  exposed  to  strong  sunlight  for  a  time,  in  a  limited 
quantity  of  water,  bubbles  of  oxygen  gas  were  seen  to  form  within  the  cavity 
of  the  polyps  and  then  to  escape  through  the  mouth.  The  production  of 
oxygen  is  without  doubt  dependent  upon  the  metabolic  activity  of  the  uni- 
cellular algae  or  Zooxanthellse  present  in  great  numbers  throughout  the 
endodermal  tissues  (p.  22).  The  filamentous  green  algae  found  perforating 
the  skeleton  (p.  38)  would  doubtless  have  a  similar  activity,  but  the  intensity 
of  the  light  reaching  their  chloroplasts  must  be  much  less  than  that  falling 
upon  the  more  superficial  Zooxanthellae. 

The  oxygen  produced  from  the  combination  of  the  physiological  activities 
of  the  plant  and  animal  was  ample  to  keep  a  limited  quantity  of  water  in  a 
proper  state  of  aeration ;  and  once  the  colonies  were  established  the  water  in 
which  they  were  living  remained  perfectly  clear  and  sparkling  without  the 
addition  of  other  plant  life.  Generally  it  was  unnecessary  to  renew  the  sea- 
water,  but  from  time  to  time  fresh  water  was  added  to  replace  that  lost  by 
evaporation.  To  make  certain  that  the  water  contained  sufficient  calcareous 
salts  in  solution  to  enable  the  young  polyps  to  build  up  their  skeleton  it 
was  partly  changed  every  few  days.  Like  the  adult  polyps,  the  developing 
individuals  contained  many  Zooxanthellae  within  the  endoderm. 

For  food  the  polyps  were  given  small  living  worms,  pieces  of  molluscs, 
crabs,  or  fish.  These  they  took  readily  by  first  extending  their  tentacles 
and  later  their  mouths  over  them.  Sometimes  a  hapless  polychaete  would 
attempt  to  creep  over  a  colony,  when  it  was  quickly  killed  and  ingested ;  other 
small  worms,  wriggling  with  great  vigor,  were  unable  to  free  themselves  when 
dropped  upon  a  colony.  The  tentacles  of  the  polyps  fastened  upon  the 
annelid  at  one  or  more  places,  and  its  movements  soon  began  to  weaken, 
as  if  the  creature  were  paralyzed.  Bach  polyp  immediately  around  the 
worm  opened  its  mouth  widely,  while  those  more  distant  extended  their  per- 
istomes towards  it,  as  if  all  were  moved  by  a  common  impulse.  As  many  as 
half  a  dozen  polyps  would  thus  seize  hold  of  a  small  worm  at  one  time. 
Naturally  a  difficulty  arose  when  two  polyps  commenced  to  engulf  the  same 
worm  from  opposite  ends.  The  unfortunate  annelid  soon  reached  a  state 
of  tension  between  the  polyps,  and  the  peristome  of  each  of  the  latter  was 
drawn  outwards  to  an  unusual  degree.     For  over   an   hour  a  worm  was 


6  SIDERASTREA    RADIANS. 

seen  thus  pulled  at  from  each  extremity.  A  large  piece  of  a  mollusc  might 
also  be  seized  upon  and  partly  ingested  by  two  polyps  at  the  same  time,  and 
in  the  end  their  lips  would  approach  and  come  into  actual  contact.  A  larval 
polyp  only  four  weeks  old  was  able  to  hold  a  wriggling  fragment  of  a  small 
worm  and  attempted  to  swallow  it* 

For  examination  of  their  anatomy  and  histology  some  of  the  polyps  were 
preserved  in  the  partly  expanded  condition  in  5  per  cent,  fomiol  and  others  in 
a  solution  of  corrosive  acetic,  after  narcotization  with  menthol  or  magnesium 
sulphate.  Shortly  after  preservation  they  were  transferred  to  alcohol.  Where 
decalcification  was  necessary  this  was  carried  out  by  means  of  weak  solutions 
of  hydrochloric  or  nitric  acid,  a  few  drops  being  added  from  time  to  time  to 
maintain  a  slight  effervescence.  For  such  a  dense  corallum  as  that  of  Sider- 
astrea  the  process  of  decalcification  required  three  or  four  days.  After  the 
process  the  space  heretofore  filled  by  the  skeleton  was  invariably  found  to 
be  occupied  by  a  fluffy  mass,  which  on  examination  under  the  microscope 
was  found  to  consist  of  delicate  algal  filaments  along  with  fine  organic 
particles.  Microscopic  sections  of  the  corallum  were  made  by  embedding  in 
Canada  balsam  and  then  grinding  to  sufficient  thinness,  as  in  the  preparation 
of  ordinary  rock  sections. 

^  *  Experiments  conducted  since  this  account  was  sent  to  the  printer  demonstrate  that  the  mucus  exuded 

^^  "  bj  the  ectoderm  cells  plays  an  important  part  in  the  feeding  of  coral  polyps,  particularly  where  the  food 
particles  are  small.  The  latter  falling  on  the  polypal  surface  become  embedded  in  mucus,  and  if  not  revers- 
ing the  ordinary  direction  of  movement  of  the  stomodaeal  cilia  they  are  slowly  wafted  awa^'  beyond  the 
tentacles ;  nutritive  particles  and  solutions  bring  about  a  reversal  of  the  action  of  the  stomodseal  cilia,  and, 
along  with  mucus  in  the  form  of  strands  or  threads,  are  gradually  drawn  into  the  polypal  cavity. 


ADULT   COLONY. 

EXTERNAL   CHARACTERS. 

The  polyps  are  small  and  subpolygonal  in  ontline,  rarely  exceeding 
4  mm.  in  diameter  when  retracted,  and  5  or  6  mm.  on  expansion.  Six  unequal 
sides  constitute  the  basal  periphery  of  large  polyps,  while  there  are  only  four 
or  five  in  smaller  examples.  In  general,  the  individuals  in  any  colony  are 
irregularly  disposed.  In  some  regions  an  arrangement  in  parallel  rows  can 
be  made  out,  but  usually  this  is  interfered  with  by  the  intercalation  of  new 
polyps  among  those  fully  grown.  In  most  branching  corals,  alcyonarians, 
and  compound  actinians,  it  is  usual  for  the  principal  oral  axis  of  the  polyp 
to  be  symmetrically  placed  with  regard  to  the  axis  of  the  colony,  but  no  such 
regularity  of  orientation  is  manifest  in  the  polyps  of  Siderastrea. 

The  superficial  polypal  tissues  are  so  thin  that  the  white  corallum 
below  shows  through,  and  occasionally  the  internal  arrangement  of  the 
mesenteries  around  the  stomodseum  can  be  followed.  This  is  especially 
the  case  with  polyps  which  have  been  preserved  in  formalin.  Retracted 
polyps  usually  exhibit  radiating  lighter  areas,  which  correspond  with  the 
cycles  of  septa  below,  and  narrower  alternating  darker  areas,  which  corre- 
spond with  the  interseptal  spaces  and  internal  attachment  of  the  mesenteries 
(plate  6,  fig.  32). 

The  living  polyps  making  up  a  colony  are  separable  from  one  another 
along  a  narrow  polygonal  groove  which  is  always  lighter  in  color  than  the 
rest  of  the  polypal  wall.  Along  this  groove  the  column  wall  is  connected 
with  the  corallum,  not  directly,  but  by  the  intermediation  of  the  mesenteries, 
and  to  whatever  degree  the  polyps  become  expanded  the  wall  is  here  never 
much  elevated.  The  polyps,  however,  are  at  no  time  raised  far  above  the 
corallum.  On  full  expansion  they  rarely  assume  the  true  cylindrical  form 
with  a  flattened  disc,  like  most  coral  polyps,  but  exhibit  merely  a  dome-like 
elevation  of  the  walls  over  the  calice,  2  or  3  mm.  high  (plate  6,  fig.  31). 

At  no  place,  except  in  simple  larval  polyps  and  at  the  margin  of  a 
colony,  is  the  boundary  of  the  column  wall  of  the  individual  polyp  directly 
connected  with  the  skeleton;  that  is,  it  is  nowhere  continuous  with  the 
skeletogenic  basal  wall.  The  margin  of  the  column  of  any  polyp  passes 
uninterruptedly  into  that  of  the  surrounding  polyps,  and,  as  will  be  shown 
later,  the  mesenteries  pass  from  the  column  wall  to  the  skeletogenic  tissues 
below,  leaving  narrow  interspaces,  which  place  the  digestive  cavities  of  all 
the  polyps  of  a  colony  in  communication. 

7 


8  SIDERASTREA    RADIANS. 

On  partial  retraction  the  column  wall  comes  to  rest  upon  the  corallum,* 
but  the  central  area  of  the  disc  and  the  tentacles  remain  a  little  elevated,  the 
peristome  especially  so.  On  fullest  retraction  these  also  come  to  lie  closely 
against  the  skeleton  and  follow  it  all  the  way,  nearly  to  the  bottom  of  the 
calice.  Unlike  the  polyps  of  most  corals,  the  column  wall  in  Siderastrea  is 
incapable  of  folding  over  the  tentacles  and  disc  upon  retraction ;  both  the 
column  wall  and  disc  are  simply  drawn  down  upon  the  skeletal  tissues, 
leaving  the  tentacles  and  mouth  exposed.  Where,  in  other  coral  polyps,  an 
overfolding  of  the  polypal  wall  occurs,  it  is  brought  about  by  the  action  of 
the  circular  endodermal  musculature  of  the  column  wall,  sometimes  in  the 
form  of  a  sphincter ;  but  histological  examination  reveals  that  the  columnar 
musculature  in  S.  radians  is  of  the  weakest  character. 

The  genus  Siderastrea^  in  many  respects,  is  allied  to  the  mushroom 
coral  Fungia^  both  being  included  within  the  section  Madreporaria  Fungida. 
The  genus  Fimgia  is  not  found  in  West  Indian  shallow  waters,  but  the 
various  figures  by  Dana  (1846),  Bourne  (1887,  1893),  and  Saville-Kent  (1893)) 
of  different  species  of  Fungia  found  elsewhere,  show  that  in  this  genus 
also  the  column  wall  in  retraction  is  not  drawn  over  the  disc.  The  West 
Indian  species  of  the  allied  genus  Agaricia  are  also  unable  to  overfold  the 
column  wall. 

Apparently  the  outer  surface  of  the  polyps  of  S.  radians  is  not  ciliated, 
for  light  particles  dropped  upon  it  remain  there,  and  no  ciliary  motion  is 
recognizable  when  the  living  polyps  are  examined  under  a  moderate  magni- 
fication. Similar  particles  dropped  upon  colonies  of  Manicina  areolata  are 
gently  swept  away  and  over  the  sides  in  such  a  manner  as  to  plainly  indicate 
ciliary  activity,  so  that  probably  different  coral  species  vary  in  this  respect. 
The  outer  surface  is  always  uniformly  ciliated  in  larvse  (plate  i,  fig.  i). 

COLUMN  WALL. 

The  columnar  and  discal  areas  of  the  polyps  are  not  sharply  separated 
from  one  another  under  any  condition  of  expansion  or  retraction.  The  outer 
cycle  of  tentacles,  which  in  ordinary  cases  marks  the  peripheral  boundary  of 
the  disc,  here  forms  but  an  irregular  circle,  and  its  members  are  comparatively 
widely  apart  (plate  6,  figs.  31,  32).  Including  as  column  all  that  part  of  the 
polypal  wall  between  the  polygonal  line  of  attachment  of  the  polyps  to  one 
another  and  the  outermost  cycle  of  tentacles,  its  extent  is  very  limited,  being 

♦The  free  superficial  regions  of  the  polyp  (column  and  disc)  never  come  into  actual  contact  with  the 
skeleton,  as  the  living  parts  of  the  latter  are  always  covered  with  the  basal  skeletotrophic  tissues.  When 
they  are  said  to  rest  upon  the  corallum  the  tissues  lining  the  corallum  always  intervene. 


ADULT    COLONY.  9 

represented  by  a  zone  not  exceeding  2  mm.  across.  When  the  polyps  are 
expanded  the  proximal  region  of  the  column  remains  somewhat  polygonal  in 
outline,  but  distally  it  becomes  circular  in  section,  rarely  assuming  the  tnie 
columnar  form. 

The  column  does  not  present  the  two  topographical  subdivisions,  calicinal 
and  pericalicinal,  like  most  corals.  Instead  it  extends  directly  from  the  edge 
of  the  calice,  so  that  it  is  wholly  within  it ;  in  other  words,  no  "  Randplatte  " 
or  "  edge-zone  "  occurs  in  Siderastrea. 

Structurally,  the  column  wall  is  very  thin,  partly  transparent,  and 
smooth  ;  on  expansion  vertical  lines  are  clearly  visible,  indicating  the  internal 
attachment  of  the  mesenteries. 

TENTACLES. 

The  oral  disc  is  divisible  into  an  outer  tentaculiferous  region  and  an 
inner  naked  area  or  peristome,  with  the  mouth  in  the  middle.  The  tentacles 
are  arranged  widely  apart,  and  thus  occupy  a  comparatively  broad  zone  of 
the  disc,  leaving  only  a  very  restricted  smooth  central  area.  As  can  be  seen 
from  plate  6,  fig.  32,  the  tentacular  region  during  retraction  includes  the 
greater  proportion  of  the  exposed  part  of  the  polyp,  while  the  columnar  area 
is  very  narrow,  and  little  remains  of  the  naked  portion  of  the  disc.  This 
exceptional  relationship  of  the  columnar,  tentacular,  and  discal  areas  results 
from  the  wide  intervals  which  separate  the  different  cycles  of  tentacles,  a 
similar  condition  being  apparently  characteristic  of  the  Fungidae  generally. 
In  most  coral  and  actinian  polyps  the  tentacular  cycles  are  placed  close 
together,  the  bases  of  the  tentacles  of  one  cycle  being  partly  embraced  by  the 
bases  of  the  next  cycle,  and  thus  the  tentacular  zone  is  very  narrow  compared 
with  the  remainder  of  the  disc. 

The  tentacles  of  5.  radians  are  remarkable  among  all  West  Indian 
coral  polyps  on  account  of  their  form  and  arrangement.  Under  certain  con- 
ditions of  partial  or  even  full  expansion  of  the  polyps  the  organs  can  be 
discerned  with  a  lens  as  small  sessile  tubercles,  arranged  only  approximately 
in  cycles,  some  as  pairs  and  others  singly,  and  corresponding  in  position 
with  the  septa.  In  most  cases  they  stand  out  as  small,  delicate,  simple  or 
bifurcated  finger-like  outgrowths  of  the  disc.  As  seen  externally  each 
appears  to  originate  over  the  central  termination  of  the  septum  with  which 
it  corresponds,  that  is,  before  the  margin  of  the  septum  commences  to  dip 
downward. 

When  the  tentacles  are  fully  extended  and  much  enlarged  they  appear 
as  shown  on  plate  7,  fig.  37.     The  stem  consists  of  a  broad  basal  part  which 


lO  SIDERASTREA    RADIANS. 

in  most  members  of  the  inner  cycles  becomes  bifurcated  a  little  above  midway, 
each  half  terminating  in  a  light-colored  knob  which  represents  a  battery  of 
nematocysts  ;  the  outermost  members,  however,  are  simple,  like  those  of  the 
majority  of  coral  polyps.* 

In  studying  the  internal  anatomy  of  the  polyp,  it  is  found  that  the  bifur- 
cated members  communicate  with  the  entocoelic  chambers,  and  the  simple 
forms  with  the  exocoelic  chambers.  Development  shows  that  the  double 
tentacles  are  at  first  simple,  and  such  may  be  the  condition  of  some  of  the 
outer  entocoelic  in  nearly  mature  polyps.  The  number  of  tentacles  always 
corresponds  with  the  number  of  septa,  the  bilobed  tentacles  being  situated 
over  the  entosepta,  and  the  simple  tentacles  over  the  exosepta.  Their  order 
of  appearance  is  described  later  in  connection  with  the  development  of  the 
larval  polyps. 

The  tentacular  walls  are  smooth,  that  is,  devoid  of  the  nematoblast 
tubercles  which  so  frequently  characterize  the  tentacles  of  coral  polyps. 
The  aggregation  of  large  nematoblasts  is  restricted  to  the  apical  knobs. 
The  stems  are  hollow  and  brown  in  color,  the  latter  character  due  to  the 
presence  of  Zooxanthellae  in  the  endodermal  tissues.  The  knobs,  on  the 
other  hand,  are  solid  and  colorless.  The  organs  are  adhesive  and  able  to 
take  particles  of  food  from  the  tips  of  forceps.  There  is  little  difference  in 
size  among  the  tentacles  of  the  various  cycles,  but  the  bifurcated  members 
are  larger  than  the  simple. 

During  retraction,  and  even  sometimes  on  full  expansion  of  the  polyp, 
the  tentacles  appear  as  unstalked,  spheroidal  tubercles,  sessile  on  the  disc. 
In  the  case  of  the  bilobed  tentacles,  a  knob  is  situated  on  each  side  of  the 
septum  with  which  the  tentacle  corresponds,  while  in  the  simple  tentacles  the 
knob  is  directly  over  the  septum  (plate  6,  fig.  32).  No  stem  can  be  recog- 
nized under  such  circumstances,  and  sections  show  that  it  is  not  invaginated, 
but  has  become  part  of  the  discal  wall  (plate  7,  fig.  40).  Such  a  condition 
of  the  tentacular  stems  is  often  found  in  corals,  but  rarely  among  the  larger 
tentacles  of  actinians.  When  the  tentacles  are  fully  expanded  they  stand 
obliquely  to  the  surface  of  the  disc,  and  are  bent  outwardly.  The  organs 
have  never  been  found  invaginated,  a  phenomenon  which  frequently  occurs 
in  coral  polyps  and  alcyonarians. 

*  Verrill  (1901,  p.  154)  makes  the  following  remarks  with  regard  to  the  tentacles  of  5.  radians  :  "  the 
tentacles  are  small,  short,  cylindrical,  orclavate;  they  form  several  circles,  and]appear  somewhat  scattered 
those  of  successive  cycles  being  in  different  circles  and  decreasing  in  size.  But  they  are  not  bilobed,  ?ior 
trilobed,  as  Agassiz  and  Pourtales  supposed.  This  appearance  is  due  to  a  smaller  one  standing  on  one 
or  both  sides  of  a  larger  one,  and  close  to  it."  From  the  part  italicized  it  would  appear  that  the  tentacles 
in  Verrill's  Bermudan  specimens  must  have  been  altogether  different  from  those  in  Jamaica  polyps, 
which,  moreover,  agree  with  those  observed  by  Agassiz  and  Pourtales. 


ADULT    COLONY.  II 

The  liexameral  cyclic  arrangement  of  tlie  tentacles  is  by  no  means  easy 
to  establish,  and  would  be  almost  impossible  to  ascertain  without  anatomical 
aid  or  assistance  from  the  septa  below.  According  to  the  usually  accepted 
cyclic  formula  for  hexactinian  polyps  employed  in  systematic  works,  S. 
radians  would  be  said  to  have  three  cycles  complete  and  a  fourth  incomplete, 
and  the  cyclic  formula  would  be  6,  6,  12,  x,  where  x  may  be  any  number 
from  I  to  24. 

On  the  living  polyp  six  bifurcated  tentacles  can  usually  be  distinguished 
as  constituting  an  inner  or  primary  cycle,  which  is  widely  separated  from 
the  other  cycles ;  the  remaining  tentacles  are  more  closely  disposed  and  their 
arrangement  is  somewhat  obscure,  simple  and  double  members  being  inter- 
mingled. Among  them  a  second  cycle  of  six  bilobed  tentacles  can  sometimes 
be  recognized,  alternating  with  the  first  and  remote  from  it.  Beyond  this 
there  is  no  true  hexameral  plan  determinable.  In  most  polyps  a  few  mem- 
bers of  a  third  cycle  of  bilobed  tentacles  are  present,  but  the  full  complement  of 
twelve  entotentacles,  necessary  by  the  laws  of  actinian  symmetry  to  complete 
the  alternation  with  the  two  previous  cycles  of  six  each,  appears  never  to  be 
developed.  The  outer  simple  exoccelic  tentacles  correspond  in  number  with 
the  sum  of  the  members  of  the  primary,  secondary,  and  tertiary  entotentacles 
with  which  they  alternate,  but  some  belong  to  the  third  cycle  and  some  to 
the  fourth.  Only  the  first  and  second  cycles  are  completely  hexameral ; 
the  third  and  fourth  are  incompletely  so. 

In  transverse  sections  through  the  polyps  (plate  6,  fig.  34)  it  is  found 
that  the  first  and  second  cycles  of  mesenteries  consist  of  six  pairs  each,  but  the 
third  cycle  rarely  or  never  contains  the  next  number  in  the  hexameral  sequence, 
namely,  twelve.  The  number  of  entocoelic  chambers  corresponds  with  the 
number  of  exocoelic  chambers,  therefore  the  number  of  entotentacles  will 
correspond  with  the  number  of  exotentacles,  as  a  tentacle  communicates  with 
each  mesenterial  chamber.  Whatever  number  of  entotentacles  be  lacking 
to  complete  the  third  cycle  of  12,  a  like  number  of  exotentacles  will  be 
wanting  to  complete  the  fourth  cycle  of  24.  Thus,  considered  morpho- 
logically, the  tentacular  formula  will  be  6,  6,  x,  6  -f-  6  +  x,  where  x  will 
be  the  same  in  the  two  cases,  and  may  be  any  number  from  i  to  12  ;  the 
series  6,  6,  x  will  represent  the  number  of  entotentacles,  and  6  -|-  6  +  x  the 
number  of  exotentacles.  A  polyp  with  38  tentacles  will  have  the  morpho- 
logical formula  6,  6,  7,  19 ;  a  polyp  with  44  tentacles  the  formula  6,  6,  10,  22. 

Where,  in  mature  polyps,  the  hexameral  sequence  is  incomplete,  it  seems 
preferable  to  employ  the  morphological  formula  as  compared  with  the  ordinary 
cyclic  formula.     In  the  first  we  have  the  true  nature  of  the  tentacles  indicated ; 


12 


SIDERASTREA    RADIANS. 


in  the  second  the  penultimate  cycle  is  assumed  to  be  complete,  whereas  it  is 
made  up  of  two  different  series  of  tentacles — entotentacles  and  exotentacles — 
and  simply  represents  an  arrested  stage  in  the  hexameral  plan. 

Fig.  I  is  a  diagrammatic  representation  of  the  arrangement  of  the  tenta- 
cles in  a  polyp  of  Siderastrea  with  the  morphological  enumeration  added. 
In  the  two  inner  cycles  (i,  ii)  six  bilobed  tentacles  are  complete,  while  only 
five  tentacles  of  the  third  cycle  (iii)  are  yet  developed ;  alternating  with  the 
members  of  the  three  inner  cycles  of  bifurcated  tentacles  is  a  simple  tentacle 


IllaX    I  X    Ilia. 


Fig.  1. — Diagram  showing  the  relationships  of  the  septa  and  tentacles  in  the  same 
polyp  as  that  from  which  fig.  34,  plate  6,  was  taken.  All  the  entotentacles  are 
shown  with  a  double  apex  and  the  exotentacles  with  a  single  apex.  The  exo- 
tentacles are  all  arranged  as  if  forming  the  fourth  or  outermost  cycle,  whereas 
some  will  belong  to  the  third  and  some  to  the  fourth  cycle,  according  as  a  third 
cycle  entotentacle  is  present  or  not. 

(x)  belonging  to  the  outermost  series,  but  not  forming  a  true  cycle.  The  septa, 
indicated  by  the  thick  radiating  lines,  show  a  corresponding  arrangement. 
For  purposes  of  comparison  the  same  polyp  is  also  shown  in  transverse  section 
on  plate  6,  fig.  34,  while  the  septa  are  diagrammatically  represented  in  fig.  2, 
p.  13.  By  comparing  figs,  i,  2,  and  plate  6,  fig.  34,  it  is  seen  that  the  three 
inner  cycles  of  tentacles  communicate  with  the  entocceles  and  correspond  with 
the  entosepta,  while  the  members  of  the  outermost  cycle  communicate  with 
the  exocoeles  and  correspond  with  the  exosepta. 

The  tentacles  of  S.  siderea  are  of  the  same  character  as  those  of  S.  radians^ 


ADULT    COLONY. 


13 


only  somewliat  larger ;  the  entotentacles  are  bifurcated,  and  tlie  exotentacles 
simple.  In  the  genus  Agaricia^  also  belonging  to  the  Madreporaria  Fungida, 
the  tentacles  are  all  simple,  but  in  their  wide  distance  apart  they  resemble 
those  of  Siderastrea. 

The  fully  expanded  tentacles  of  most  species  of  Fungia  seem  to  be  small 
and  club-shaped,  but  in  F.  crassitentaculata  they  are  much  larger  than  is 
usual  for  the  organs  in  corals  generally.     This  is  well  shown  in  the  figure 


Fig.  2. — Diagram  showing  the  relationship  of  the  septa  to  one  another  in  the  same 
polyp  as  that  of  fig.  i. 


oi  F.  crassitentaculata  in  Saville-Kent's  "  Barrier  Reef."  In  all  Fungia  the 
organs  are  widely  apart,  and,  as  in  Siderastrea  and  Agaricia^  occupy  nearly 
the  whole  discal  area. 

The  wide  separation  of  the  tentacles  in  the  genera  mentioned  usually 
tends  to  obscure  their  hexameral  cyclic  plan.  The  two  polyps  of  Siderastrea 
represented  on  plate  xv  in  Agassiz's  "  Florida  Reefs  "  are  evidence  of  this, 
and  also  the  remarks  of  Verrill,  given  in  the  foot-note  on  p.  10.  Such  is  also 
the  case  with  Dana's  figure  of  Fungia  lacerta^  but  Bourne  found  the  tenta- 
cles of  F.  clavata  to  be  arranged  in  distinct  cycles.  The  positions  usually 
assumed  by  the  tentacles  of  Siderastrea  enable  one  to  understand  how  such 


14  SIDERASTREA     RADIANS. 

confusion  may  arise ;  indeed,  the  polyps  were  for  a  long  time  under  observa- 
tion before  any  regular  arrangement  could  be  established  with  certainty. 
It  was  only  after  observing  the  Jamaica  polyps  under  various  phases  of 
expansion  that  the  cyclic  plan  of  the  first  tentacles  could  be  determined,, 
while,  as  regards  the  two  outer  cycles,  such  an  arrangement  is  more 
theoretical  than  actual.  The  tentacles  on  plate  6,  fig.  32,  from  a  camera 
drawing  of  a  preserved  polyp,  do  not  readily  suggest  a  hexameral  cyclic 
plan. 

Siderastrea  is  apparently  the  only  madreporarian  genus  with  true 
dimorphic  tentacles,  the  peculiarity  having  been  first  recognized  by  Agassiz. 
The  independent  origin  of  the  two  moieties  of  the  bifurcation,  as  described 
later,  is  also  a  remarkable  feature  in  the  tentacular  development  of  the 
Zoantharia.  Dimorphic  tentacles  are  occasionally  met  with  among  the 
tropical  Actiniaria,  e.  g.^  Rhodactis^  Phymanthus ;  but  here  they  are  the  disc 
tentacles  as  compared  with  the  marginal  tentacles  which  show  the  distinc- 
tion, and  the  morphological  value  of  the  former  is  uncertain.  At  any  rate, 
the  disc  and  marginal  tentacles  of  actinians  are  not  comparable  with  the 
entotentacles  and  exotentacles  of  Siderastrea. 

DISC  AND  MOUTH. 

The  naked  portion  of  the  disc  is  very  limited  in  extent.  It  is  smooth, 
circular,  and  usually  partly  depressed  within  the  calice.  So  transparent  is 
the  wall  that  the  cyclic  arrangement  of  the  internal  mesenteries  and  septa 
can  be  easily  followed.  The  central  part  or  peristome  is  not  separable  from 
the  rest  of  the  disc,  except  that  it  is  usually  elevated  in  a  conical  manner,  and, 
owing  to  a  greater  concentration  of  the  endodermal  Zooxanthellse,  it  appears 
darker  than  the  rest  of  the  polyp. 

The  mouth  is  small  and  slit-like  when  closed,  oval  or  circular  when 
open.  The  lips  are  not  specially  marked  off  from  the  peristome  by  any 
tumidity,  such  as  occurs  in  many  anemones.  In  living  polyps  the  mouth 
sometimes  closes  along  the  middle  by  the  approximation  of  the  lips,  but 
leaves  a  small  opening  at  each  end.  The  apertures,  however,  can  not  be 
regarded  as  representing  a  siphonoglyph  or  gonidial  groove,  for  microscopic 
examination  reveals  no  difference  in  the  character  of  the  stomodasal  wall 
between  the  sides  and  extremities.  On  narcotization  the  circular  muscles  of 
the  disc  relax,  and  the  oral  aperture  remains  widely  open  and  circular;  the 
stomodseal  wall  is  then  seen  to  be  smooth,  without  ridges  and  grooves. 


ADULT    COLONY.  1 5 

COLOR. 

The  polyps  on  the  under,  unexposed  surface  of  colonies  may  be  color- 
less, but  vary  elsewhere  from  light  to  dark  brown.  Owing  to  the  partial 
transparency  of  the  tissues  the  white  corallum  always  shows  through,  and 
gives  rise  to  an  alternation  of  light  and  dark  radiating  areas  and  to  a  light 
polygonal  line  separating  one  polyp  from  the  other.  Sometimes  the  disc 
presents  a  velvety  green  color  along  the  mesenterial:  radii,  and  the  oral  aspect 
of  the  tentacles  and  the  angles  of  the  mouth  also  may  be  of  the  same  color. 
The  stomodasal  walls  and  knobs  of  the  tentacles  are  white,  while  the  stems 
of  the  latter  are  usually  brown.  More  rarely  the  elevated  peristome  may  be 
rose-colored,  and  in  some  cases  purple.* 

The  different  shades  of  brown  exhibited  by  the  polyps  are  entirely  due 
to  the  presence  of  Zooxanthellae  or  unicellular  algse  within  the  endodermal 
layer.  The  chromoplasts  within  these  are  brown,  yellow,  or  greenish  yellow, 
and  when  the  algse  are  but  few  in  number  they  give  a  light  brown  color  to 
the  walls  of  the  polyps,  whereas  when  present  in  great  numbers,  overlying 
one  another,  the  pol3rps  appear  a  dark  brown.  For  this  reason  the  thin 
tissues  of  fully  expanded  polyps  are  much  lighter  in  color  than  the  tissues 
of  retracted  polyps. 

Examination  of  the  walls  of  colorless  polyps  on  the  under  surface  of  a 
colony  proves  that  the  Zooxanthellae  are  either  absent  or  occur  very  sparingly 
in  the  endoderm.  When,  however,  the  colorless  areas  are  exposed  to  light, 
the  polypal  tissues  begin  slowly  to  assume  a  brownish  color,  and  in  a  few 
days  yellow  cells  are  to  be  found  in  abundance. 

On  examining  a  living  tentacle  under  the  microscope  the  outer  ectoder- 
mal layer  is  seen  to  be  quite  colorless,  but  the  yellow  Zooxanthellae  are 
revealed  by  focusing  through  to  the  internal  endodermal  layer.  The  white- 
ness of  the  spheroidal  tips  of  the  tentacles,  compared  with  the  brown  color  of 
the  stems,  is  also  due  to  the  fact  that  the  knobs  are  wholly  formed  of  ectoderm 
the  endoderm  lining  the  cavity  of  the  stems  not  being  prolonged  into  them. 
The  superficial  green  or  purple  color  is  somewhat  evanescent,  and  wholly 
ectodermal  in  origin. 

Different  shades  of  yellowish  brown  are  the  prevailing  colors  of  the  coral 
areas  in  Jamaica,   and  whether  in  the  palmate  or  branching  growths  of 


♦  Le  Sueur  (1820,  p.  286)  describes  the  color  of  the  specimens  seen  by  him  at  Guadeloupe  as  "  d'un 
rouge  m&16  de  violet."  The  color  of  5.  siderea  as  given  by  the  same  author  is  also  very  different  from 
that  of  the  Jamaica  specimens.  Verrill  (1901,  p.  154)  writes  thus  of  the  Bermuda  5.  radians:  "The 
general  color  in  life  is  dull  gray,  yellowish  gray,  ocher-yellow,  or  rusty  brown,  sometimes  tinged  with  a 
purplish  rosy  tint;   the  polyps  are  paler,  with  the  lips  and  tips  of  the  tentacles  whitish." 


l6  SIDERASTREA    RADIANS. 

Acropora  {Madrepoj^a) ,  the  hydrozoan  Millepora^  or  the  zoanthid  Palythoa^ 
all  owe  their  origin  to  the  presence  of  unicellular  algae  with  yellow  chromo- 
plasts  within  the  endodermal  cells.* 

Stanley  Gardiner  and  Hickson  have  both  drawn  attention  to  the  part 
which  commensal  algae  probably  play  in  the  nutrition  of  the  polyp.  The 
organisms  are  actually  inclosed  within  the  endodermal  cells,  and  there  can 
be  scarcely  any  doubt  that  the  polyp  receives  from  them  nutritive  carbohy- 
drates, produced  in  the  course  of  the  metabolic  activity  of  the  vegetable  cells 
under  the  action  of  sunlight ;  further,  the  Zooxanthellae  are  constantly 
increasing  in  number  by  fission  into  two  and  then  four,  and  perhaps  some 
are  utilized  as  food  directly  by  the  polyp.  The  liberation  of  free  oxygen, 
already  shown  to  take  place  by  the  activity  of  the  chlorophyll  within  the 
algal  cells,  must  also  be  of  importance  in  the  vital  activities  of  the  polyps. 

In  many  corals  and  actinians,  however,  Zooxanthellae  are  altogether 
absent,  hence  their  presence  can  not  be  considered  as  necessary  to  the  life 
of  the  polyp ;  also,  corals  grown  in  the  shade  are  colorless  from  absence  of 
the  algae.  Further,  both  actinians  and  corals  will  ingest  almost  any  variety 
of  animal  food  that  is  offered  them. 

REPRODUCTION. 

New  polyps  arise  asexually  as  intercalary  buds,  usually  at  the  point  of 
junction  of  three  or  more  polyps,  so  that  it  is  impossible  to  say  whether  one 
polyp  more  than  another  is  to  be  regarded  as  the  parent,  or  as  to  how  far 
the  structures  in  the  bud  arise  in  organic  association  with  those  of  the  older 
polyps.  Over  the  general  surface  of  any  colony  bud  polyps  in  different 
stages  of  development  are  usually  present,  and  for  some  time  they  remain 
much  lighter  in  color  than  the  others,  owing  to  a  less  growth  of  Zooxan- 
thellae ;  also,  around  the  margin  of  colonies  the  addition  of  new  polyps 
is  generally  in  rapid  progress.  These  latter  serve  to  enlarge  the  lateral 
boundaries  of  the  colonies,  while  the  intercalary  polyps  occupy  the  spaces 
produced  as  the  coral  grows  in  height,  and,  of  course,  in  superficial  area. 
Instances  of  reproduction  by  the  process  known  as  fissiparous  gemmationf 

*  In  his  recent  vice-presidential  address  before  the  Section  of  Zoology  of  the  American  Association 
at  St.  Louis,  Prof.  C.  W.  Hargitt  comes  to  the  conclusion  that  the  colors  in  coelenterates  and  many  other 
groups  of  invertebrates  have  probably  no  adaptive  or  protective  significance,  and  are  in  main  the  result 
of  the  metabolic  activity  of  the  animal.  The  rich  profusion  and  beauty  of  color  in  coral  polyps  certainly 
Beems  to  have  no  protective  or  warning  significance,  but  those  due  to  the  presence  of  commensal  alg« 
have  of  course  an  altogether  different  physiological  importance  from  the  other  colors. 

t  The  term  is  applied  to  a  method  of  asexual  reproduction  occasionally  found  in  gemmiferous  colonies. 
It  appears  as  if  an  enlarged  polyp  simply  divides  into  two.  The  method,  however,  is  altogether  different 
from  growth  in  fission  colonies  generally.  ("The  Morphology  of  the  Madreporaria — IV.  Fissiparous 
Gemmation,"    Ann.  Mag.  Nat.  Hist.,  ser.  7,  vol.  xi,  1903.) 


ADULT    COLONY.  1 7 

are  very  rare,  but  occasionally  much  enlarged  calices  are  seen  undergoing 
binary  fission.  The  detailed  study  of  the  development  of  young  bud  polyps 
has  not  been  carried  out,  as  the  species  is  unsuited  for  the  purpose.  It  has 
been  established,  however,  that  the  manner  of  appearance  of  the  tentacles, 
mesenteries,  and  septa  agrees  closely  with  that  of  larval  polyps  to  be 
described  later. 

In  what  seem  to  be  dead  parts  of  a  colony,  new  polyps  may  arise 
within  the  old  calices.  Outwardly  these  polyps  are  at  first  wholly  sepa- 
rated from  one  another  and  from  the  other  living  tissues  of  the  colony,  and 
are  usually  transparent  and  colorless,  due  to  an  absence  of  Zooxanthellae. 
Similar  renewals  of  growth  have  been  found  in  other  corals,  and  give  rise  to 
the  successive  deposits  of  skeletal  matter  on  dead,  corroded  surfaces,  some- 
times met  with  when  masses  of  coral  are  broken  across.  The  continuity  of 
any  one  corallite  is  frequently  maintained  throughout  the  vertical  extent  of 
the  mass,  notwithstanding  the  corroded  surfaces  here  and  there. 

The  renewal  of  the  polyps  within  old  corroded  calices  of  a  coral  is  a 
subject  for  further  investigation.  So  far  as  could  be  made  out  from  external 
obser\ation  alone  such  polyps  in  Siderastrea  are  altogether  independent  of 
one  another,  and  also  unconnected  with  other  living  tissues  of  the  colony. 
It  may  be  that  within  the  deepest  parts  of  the  old  calices  there  still  remained 
living  remnants  of  the  original  polyps,  and  that  from  these  new  polyps  were 
formed  as  buds ;  on  reaching  the  same  size  as  the  original  polyps  the  buds 
would  fuse  with  one  another  by  their  peripheral  borders  and  present  a 
continuous  covering  of  soft  tissues.  New  skeletal  deposits  would  cover 
over  the  old  surface,  and  the  presence  of  the  latter  would  afterwards  be  deter- 
minable only  on  breaking  the  colony  across. 

Renewals  of  this  character,  by  the  production  of  independent  buds,  are 
to  be  distinguished  from  the  growth  at  the  living  margin  of  colonies  which 
is  frequently  seen  encroaching  over  dead  areas.  The  latter  is  of  the  same 
character  as  ordinary  marginal  gemmation  from  the  parent  stock,  even 
though  it  extends  over  old  corroded  surfaces.  Such  new  growth  is  very 
frequent  in  branching  stocks  of  Acropora^  but  may  occur  in  almost  any 
colonial  species. 

The  death  of  the  polyps  over  any  restricted  area  of  a  colony  may  be 
brought  about  by  many  causes,  such  as  the  colony  becoming  partly  covered 
by  or  embedded  in  sand  or  mud,  or  by  adherence  or  contact  with  other 
foreign  bodies,  or  by  exposure.  Local  death  is  very  frequent  in  such  large 
hemispheroidal  colonies  as  those  of  Mceandrina  and  Orbicella  ;  the  middle 
portion  of  the  blocks  may  be  dead  and  disintegrating  while  the  sides  are  in 


1 8  SIDERASTREA    RADIANS. 

full  vigorous  growth.  In  addition  to  this,  the  death  of  colonies  as  a  whole, 
without  any  obvious  environmental  cause,  is  not  infrequent.  Stanley  Gardi- 
ner has  recently  drawn  attention  to  this  subject.  During  his  investiga- 
tions on  the  coral  reefs  in  the  Maldives  and  Laccadives  the  impression 
was  gathered  "  that  practically  all  the  colonies  of  a  species  in  any  one  area 
died,  or  that  there  were  only  the  isolated  deaths  of  individual  large  colonies," 
apparently  without  environmental  cause.  Gardiner  is  impelled  to  believe 
that  "  The  ripening  of  the  generative  organs  of  a  large  number  of  polyp 
colonies  of  the  same  species  in  a  single  locality  or  habitat,  followed  by  the 
subsequent  death  of  all  these  colonies,  is  a  regular  phenomenon." 

In  the  course  of  my  experience  in  the  coral  regions  of  the  West  Indies 
I  have  met  with  no  instance  of  the  regional  death  of  all  the  colonies  of  any 
species,  but  frequently  individual  stocks  have  been  encountered  of  which  all 
the  polyps  seemed  to  be  in  a  state  of  maceration.  This  was  particularly  the 
case  with  Forties  astrcBoides.  Isolated  colonies  were  obtained  seemingly  alive 
and  normal  in  color,  but  upon  examination  with  a  lens  no  distinct  polyps  or 
tentacles  were  recognizable.  The  whole  of  the  soft  tissues  seemed  to  be  a 
gelatinous  mass  in  process  of  decay,  the  coloration  being  due  to  the  persist- 
ence of  the  yellow  pigment  cells  characteristic  of  this  species.  Such  colonies 
were  under  exactly  similar  conditions  to  others  living  around  them,  and 
without  further  investigation  any  suggestion  as  to  the  cause  of  death  of 
the  polyps  can  be  only  the  merest  conjecture. 

My  own  experience  leads  me  to  suppose  that  coral  polyps  do  not  die 
after  the  ripening  of  the  generative  products,  but  that,  from  the  same  indi- 
vidual, one  series  of  larvae  may  follow  another,  for  in  numerous  instances 
{Favia,  Forties)  in  which  polyps  charged  with  larvae,  all  at  the  same  stage 
of  development,  have  been  examined,  there  were  still  many  nearly  ripe  ova 
within  the  mesenteries,  as  if  preparing  to  give  rise  to  another  batch  of  larvae. 

Sexual  reproduction  in  Stderastrea  takes  place  by  the  formation  of 
planulae  from  fertilized  ova.  The  planulae  undergo  partial  development,  as 
far  as  the  appearance  of  the  first  four  pairs  of  mesenteries,  while  within  the 
body-cavity  of  the  parent,  and  are  then  expelled.  After  swimming  freely  for 
a  shorter  or  longer  time  they  settle  and  give  rise  to  young  polyps.  Larvae 
were  extruded  toward  the  end  of  June  and  during  July. 

EXTERNAL  CHARACTERS  ON  DECALCIFICATION. 

Certain  of  the  external  features  of  the  polyps  can  be  studied  only  after 
the  latter  have  been  freed  from  the  skeletal  matrix.  The  corallum  of  Sider- 
asirea  is  very  dense,  and  the  process  of  decalcification  requires  several  days. 


ADULT    COLONY.  I9 

At  the  close  of  the  process  a  delicate  mass  of  algal  filaments  remains  in  the 
spaces  previously  occupied  by  the  corallum,  but  is  readily  removed.  The 
exposed  polypal  wall  then  reveals  a  smooth  surface,  very  complicated  in 
form,  but  everywhere  continuous. 

After  removal  of  the  skeleton  the  individual  polyps  are  seen  to  be 
wholly  free  from  one  another,  except  at  the  surface  of  the  colony.  No 
other  connection  or  communication  between  one  polyp  and  another  exists. 
Each  polyp  appears  as  if  made  up  of  a  deeply  lamellated  column,  attached 
above  to  a  flattened  layer  which  represents  the  column  wall  and  disc.  A 
middle  tubular  space,  formerly  occupied  by  the  columella,  extends  for  some 
distance  upwards,  when  it  terminates  blindU^  The  lamellae  represent  the 
tissues  which  occupied  the  interseptal  spaces.  They  are  now  wholly  free 
from  one  another  below,  but  in  the  upper  region  are  united  centrally.  They 
are  easily  torn  apart  along  their  central  and  upper  lines  of  attachment. 
One  of  the  separated  lamellae,  slightly  enlarged,  is  represented  on  plate  6, 

fig-  33. 

The  whole  of  the  external  wall  liberated  by  decalcification  is  the  mor- 
phological base  of  the  polyp.  Though  columnar  in  form  it  in  no  way 
corresponds  with  the  column  of  the  skeletonless  Actiniaria,  but  represents 
the  flattened  basal  disc  which,  everywhere  continuous,  has  become  much 
infolded  and  subdivided  in  correspondence  with  the  upward  growth  of  the 
septa  and  columella.  Thus  the  column  of  a  decalcified  retracted  polyp  is 
really  the  vertically  elongated,  much  subdivided  basal  disc.  The  true  column 
wall,  corresponding  with  that  of  an  actinian,  is  here  represented  by  the 
narrow  periphery  of  the  flattened  or  concave  superficial  disc  which  constitutes 
the  upper  end  of  the  polyp.  To  understand  the  form  ultimately  assumed 
by  the  originally  flat  basal  disc,  the  arrangement  of  the  septa  and  columella 
in  the  individual  corallite  must  be  borne  in  mind,  as  every  part  of  these  is 
covered  by  the  basal  wall.  An  early  stage  in  the  basal  complexity  is 
represented  in  the  section  of  the  young  polyp  on  plate  9,  fig.  53. 

The  upper  part  of  the  liberated  tissues  appears  delicate  and  somewhat 
clear  and  transparent,  but  as  the  aboral  termination  is  approached  the  walls 
become  denser  and  more  opaque  white.  Such  an  alteration  in  the  external 
character  of  the  embedded  tissues  is  met  with  in  nearly  all  corals,  and  is 
usually  more  marked  than  in  Siderastrea.  It  is  found  to  be  associated  with 
a  corresponding  thickening  and  histological  modification  of  the  endodermal 
layer  (p.  32). 

The  polyps  when  set  free  are  subpolygonal  in  outline,  and,  as  already 
mentioned,  are  wholly  distinct  from  one  another,  except  at  their  uppermost 


20  SIDERASTREA    RADIANS. 

termination,  where  they  are  all  in  communication  and  joined  along  the  poly- 
gonal boundary  lines  of  the  column  wall  and  skeletogenic  tissues  immediately 
below.  An  interval  of  only  about  i  mm.  separates  laterally  the  columns  of 
contiguous  polyps.  The  length  of  the  polyps  liberated  from  the  skeleton 
varies  somewhat,  but  whatever  the  thickness  of  the  corallum  the  polypal 
tissues  are  always  superficial,  never  extending  downward  for  more  than 
about  5  mm.  The  largest  polyps  vary  from  4  mm.  to  5  mm.  in  length  after 
decalcification,  while  young  buds  extend  much  less  within  the  corallum. 
The  diameter  also  varies  from  3  mm.  to  5  mm.,  and  in  mature  polyps  is  prac- 
tically the  same  throughout  the  column.  The  soft  tissues  of  young  buds, 
before  any  columella  has  appeared,  are  more  obconical,  that  is,  longer  centrally 
and  shortening  as  they  pass  to  the  periphery.  Mature  polyps  are  more 
cylindrical  and  terminate  somewhat  abruptly,  all  the  lamellae  being  of  prac- 
tically the  same  vertical  length.  Owing  to  the  presence  of  synapticula,  the 
actual  base  of  the  individual  lamellae  is  not  always  as  regular  or  even  as  in 
corals  resting  upon  a  smooth  dissepiment ;  the  lower  surface  of  a  lamella 
may,  in  fact,  be  partly  folded  round  a  sjmapticular  bar,  as  in  plate  6,  fig.  33. 

The  individual  interseptal  lamellae,  like  the  septa  themselves,  belong 
to  different  cycles  and  are  of  different  radial  lengths.  Some  are  simple, 
while  others  are  subdivided  peripherally  into  two  or  three  parts.  For  the 
greater  part  of  their  vertical  length,  that  is,  as  far  as  the  columella  extends, 
the  lamellae  are  free,  either  singly  or  in  groups  of  two  or  three,  but  towards 
the  upper  extremity  they  are  all  joined  to  one  another  along  their  inner 
margins  (plate  6,  fig.  34). 

Any  simple  lamella  when  seen  in  surface  view  appears  as  a  flat,  sub- 
rectangular  plate,  nearly  of  the  same  thickness  throughout,  and  correspond- 
ing, of  course,  with  an  interseptal  space  (plate  6,  fig.  33).  It  is  perforated 
by  rows  of  circular  or  oval  apertures,  arranged  roughly  in  one  to  four  vertical 
series,  the  number  varying  with  the  cycle  to  which  the  lamella  belongs. 
In  lamellae  extending  all  the  way  from  the  periphery  to  the  columella  three 
or  four  rows  occur,  while  in  narrower  lamellae  only  one  or  two  rows  are  present. 
The  perforations  represent  the  spaces  occupied  by  the  calcareous  synapticula 
which  stretch  across  the  interval  from  one  septum  to  an  adjacent  septum. 
Numerous  granulations  occur  on  the  septal  walls  (plate  10,  fig.  63),  but  only 
the  large  more  peripheral  ones  bridge  the  interspace  between  two  adjacent 
septa,  and,  in  doing  so,  necessarily  perforate  the  soft  tissues  lining  the  septal 
walls  and  the  mesenteries  contained  within.  The  smaller  granulations  merely 
cause  indentations  in  the  lining  tissues. 

Transverse  sections  through  the  lamellae  show  that  they  consist  of  the 


ADULT    COLONY.  21 

two  lateral  walls  which  line  the  adjacent  faces  of  two  contiguous  septa,  and 
inclose  between  them  a  narrow  chamber,  broken  up  by  the  synapticular 
perforations  in  such  a  manner  as  to  present  a  complex  canal-like  condition 
(plate  6,  fig.  34,  and  plate  7,  figs.  38,  39).  The  chambers  represent  the  inter. 
septal  loculi  less  the  thickness  of  the  lining  wall,  but  will,  however,  be  spoken 
of  as  interseptal  spaces.  The  upper  half  of  each  contains  a  single  mesentery, 
but  below  they  are  empty.  Moreover,  only  at  the  uppermost  extremity  of 
the  polyp  is  the  mesentery  found  to  extend  outwardly  as  far  as  the  peripheral 
boundary.  In  the  upper  region  of  the  polyp  the  inner  border  of  each  lamella 
is  open  so  that  the  chamber,  along  with  all  the  others,  communicates  with  the 
central  cavity  of  the  polyp,  while  below  all  the  chambers  are  closed  centrally, 
or  are  only  in  communication  toward  their  centripetal  edges  (plate  7,  figs. 
38,  39).  Where  the  chambers  are  wholly  cut  off  from  one  another  the 
calcareous  septa  extend  throughout  the  radial  length  of  the  polyp,  and 
are  centrally  united  with  others  or  with  the  columella  (plate  7,  fig.  39) ; 
when  the  chambers  are  only  partly  separated,  communicating  towards  the 
center,  then  the  septa  between  them  do  not  extend  as  far  as  the  middle  of 
the  calice. 

ANATOMY  AND  HISTOLOGY. 
COLUMN  WALL  AND  DISC. 

In  radial  sections  of  retracted  polyps  the  column  wall  and  disc  appear 
as  a  continuous  layer,  and,  histologically,  the  two  are  nearly  alike  through- 
out. The  wall  as  a  whole  is  very  thin,  except  where  the  knob  of  a  tentacle 
is  included,  when  the  ectoderm  becomes  thickened  (plate  7,  fig.  40,  and  plate 
9,  fig.  53);  to  a  less  degree  the  layer  is  also  thickened  along  the  line  of 
attachment  of  the  mesenteries.  In  sections  the  three  coelenterate  layers, 
ectoderm,  mesogloea,  and  endoderm,  together,  measure  only  about  0.07 
mm.  in  thickness. 

The  chief  constituents  of  the  ectoderm  of  the  column  wall  are  support- 
ing cells  and  clear  mucous  gland  cells,  while  small  nematocysts  occur  some- 
what sparsely.  The  unicellular  gland  cells  are  most  conspicuous  toward  the 
periphery  of  the  layer,  where  they  appear  oval,  with  clear  or  vesicular  con- 
tents. The  nuclei  of  the  various  cells  are  oval,  and  arranged  as  a  whole 
within  the  inner  half  of  the  ectoderm,  a  few,  more  circular  in  form,  being 
found  near  the  mesogloea.  There  is  no  trace  of  any  ectodermal  musculature 
on  the  column,  but  a  system  of  weak  fibrils  is  developed  over  the  disc,  their 
direction  being  radial.  No  evidence  of  external  ciliation  can  be  detected 
even  in  the  best  preserved  examples. 

The  mesogloea  in  the  column,  as  elsewhere  throughout  the  polyp,  is 


22  SIDERASTREA    RADIANS. 

feebly  developed.  In  the  column  and  disc  it  appears  only  as  a  thin  dividing 
lamella  between  the  ectoderm  and  endoderm,  but  in  the  mesenteries  it 
becomes  a  little  broader.  It  is  homogeneous  in  structure,  except  for  the 
presence  of  minute  connective-tissue  cells.  The  mesogloea  of  corals  gener- 
ally appears  perfectly  homogeneous,  but  in  several  West  Indian  species  the 
layer  is  found  to  contain  migrant  connective-tissue  cells,  such  as  occur  in 
the  larger  actinians. 

The  endoderm  of  the  column  ^nd  disc  is  a  little  broader  than  the  ectoderm, 
and  its  cells  contain  numerous  Zooxanthellse,  which  are  altogether  absent 
from  the  latter.  The  endodermal  cells  are  much  vacuolated,  and  distinct 
cell  outlines  can  be  made  out.  The  commensal  algse  are  more  numerous  in 
some  regions  of  the  endoderm  than,  in  others,  though  there  seems  to  be  no 
regularity  in  their  distribution.  Where  they  are  absent,  or  nearly  so,  the 
endoderm  is  somewhat  thinner,  its  cells  are  less  vacuolated,  and  the  layer  as 
a  whole  stains  more  intensely,  the  nuclei  forming  a  more  regular  band. 

A  delicate  circular  muscle  layer  can  be  discerned  over  the  inner  face  of 
the  mesogloea  of  the  column  and  disc,  but  no  concentration,  such  as  can  be 
regarded  as  constituting  a  sphincter  muscle,  occurs  at  any  region  of  the 
column.  When  studying  the  living  characters  it  was  found  that  the  polyps 
are  unable  to  fold  the  column  over  the  disc,  hence  the  presence  of  a  special 
circular  or  sphincter  muscle,  as  met  with  in  most  anemones  and  some  few 
corals,  would  be  scarcely  expected. 

In  sections  which  have  been  passed  through  Delafield's  haematoxylin 
the  deeper  part  of  the  endodermal  layer  throughout  the  polyp,  but  particu- 
larly the  epithelial  lining  of  the  mesenteries,  contains  some  substance  which 
stains  intensely,  and  has  a  very  irregular  distribution.  The  appearance,  as 
seen  in  sections  of  mesenteries,  is  represented  on  plate  8,  fig.  50,  and  in  the 
lining  of  an  interseptal  loculus  on  plate  8,  fig,  47.  Small,  irregular,  deeply 
staining  patches  lie  next  the  mesogloea  on  both  sides,  and  prolongations 
extend  for  varying  distances  among  the  endodermal  cells,  occasionally 
reaching  almost  to  the  surface.  In  tangential  or  oblique  sections  through 
the  endoderm  an  appearance  of  somewhat  irregular  longitudinal  canals  is 
presented.  The  substance  stains  and  behaves  altogether  in  the  same  manner 
as  the  contents  of  the  mucous  cells.  No  evidence  of  structure  is  presented, 
nor  is  there  any  cellular  character  suggested.  I  conceive  that  the  appearance 
is  due  to  intercellular  spaces  filled  in  the  living  condition  with  mucus  or 
some  similar  substance,  capable  of  staining  strongly  in  haematoxylin ;  per- 
haps a  hint  of  the  lymphatic  spaces  of  the  higher  animals. 

This  is  the  first  time  that  such  a  system  of  apparently  intercellular 


ADULT    COLONY.  23 

Spaces  has  been  observed  in  the  Zoantharia.  Its  appearance  in  sections  of 
Siderastrea  stained  in  hsematoxylin  is  very  characteristic,  and  its  presence 
should  be  looked  for  in  other  forms. 

TENTACLES. 

When  the  polyps  are  preserved  in  the  retracted  condition,  with  the 
tentacles  sessile,  the  columnar  and  discal  walls  come  to  lie  upon  the  skele- 
totrophic  layer  of  the  septal  edges,  and  the  two  ^nobs  of  the  bifurcated 
tentacles  are  far  apart,  one  on  each  side  of  a  septal  ridge  (plate  7,  fig.  40), 
while  the  knob  of  the  simple  tentacles  lies  immediately  over  the  septum  with 
which  it  corresponds.  In  this  condition  the  stem  of  the  tentacle  has  altogether 
disappeared  as  such,  having  become  part  of  the  disc.  As  a  result  the  tenta- 
cles are  recognizable  in  vertical  sections  of  polyps  as  mere  hemispheroidal 
thickenings  of  the  ectoderm.  In  the  case  of  the  entoccelic  tentacles  a  thick- 
ening occurs  on  each  side  of  a  septal  elevation  of  the  disc,  while  a  single 
elevation  directly  over  the  apex  of  the  septum  represents  an  exotentacle. 
In  the  thickenings  long  nematocysts  extend  nearly  across  the  ectoderm,  and 
smaller  ones  crowd  the  periphery.  The  two  forms  of  nematocysts  are 
represented  on  plate  7,  fig.  44.  Histologically  the  portion  of  the  wall  join- 
ing the  two  tentacular  knobs  on  plate  7,  fig.  40,  and  representing  the  stem  of 
the  tentacle,  differs  in  no  way  from  that  of  the  disc. 

In  sections  of  polyps  preserved  with  the  tentacles  extended  the  organs 
appear  as  tubular  outgrowths  of  the  disc,  terminated  by  knoblike  enlarge- 
ments. The  stem  seems  to  differ  in  no  respect  histologically  from  the  disc, 
not  even  in  the  greater  number  of  nematocysts ;  the  apex  alone  constitutes  a 
battery  of  stinging  cells.  The  cut  ends  of  a  delicate  ectodermal  muscle 
layer  can  be  distinguished  in  transverse  sections  of  the  stems,  and  a  feeble 
circular  endodermal  layer  in  longitudinal  sections.  The  tentacula'r  muscu- 
lature is  continuous  with  that  of  the  disc,  and  a  very  definite  nerve  layer 
occurs  at  the  apex  of  the  tentacles  (plate  8,  fig.  49). 

STOMOD^UM. 

The  stomodaeum  is  a  short,  thin-walled,  depending  tube,  strongly  ciliated 
all  round,  and  without  any  permanent  ridges  or  grooves.  Usually  it  is 
oval  in  transverse  section,  and  along  its  endodermal  surface  six  pairs  of 
mesenteries  are  attached  at  equal  distances  apart.  Histologically  it  is  of 
the  same  structure  all  the  way  round,  there  being  no  modification  opposite 
the  directive  mesenteries  such  as  can  be  regarded  as  a  siphonoglyph  or 
gonidial  groove.  This  is  also  the  case  in  all  madreporarian  polyps  yet 
described. 


24  SIDERASTREA    RADIANS. 

The  absence  of  a  siphonoglyph  in  Madreporaria  affords  an  interesting  con- 
trast with  the  one  or  two  grooves  usually  present  in  actinian  and  alcyonarian 
polyps.  A  siphonoglyph  is  generall}^  wanting  only  in  the  lowest  actinians 
and  in  alcyonarians,  and  its  absence  in  coral  polyps  would  suggest  their 
more  primitive  nature.  This  is  further  borne  out  by  the  absence  of  ciliated 
bands  or  Flimmerstreifen  from  the  mesenterial  filaments  (p.  29). 

The  stomodseal  ectoderm  is  thicker  than  that  of  the  disc,  and  differs 
much  from  it  in  structure.  Large  nematocysts  are  distributed  throughout, 
but  the  main  constituents  are  long,  narrow,  ciliated  supportiug  cells,  the 
nuclei  of  which  are  arranged  in  a  broad  zone.  The  ciliation,  which  is  uni- 
form all  round,  generally  persists  in  preserved  material.  Clear  mucous 
gland  cells  are  not  numerous,  while  here  and  there  a  granular  gland  cell 
stands  out  very  prominently  by  reason  of  the  deeply  staining  character  of 
its  contents.  A  distinct  nerve  layer  is  recognizable  next  the  mesoglcea,  but 
no  muscular  fibrils  can  be  detected.  The  endoderm  is  similar  to  that  lining 
the  upper  part  of  the  polypal  cavity. 

At  its  lower  opening  into  the  polypal  cavity  the  ectoderm  of  the  stomo- 
daeum  is  reflected  up  the  inner  or  endodermal  surface,  and  then  extends 
horizontally  for  a  short  distance  along  the  free  edge  and  each  face  of  the 
perfect  mesenteries,  becoming  continuous  with  their  mesenterial  filaments. 

MESENTERIES. 

The  number  and  arrangement  of  the  mesenteries  in  mature  polyps  are 
as  follows :  (a)  Six  pairs  are  united  with  the  stomodseum,  including  two 
pairs  of  directives  attached  at  the  opposite  extremities ;  {d)  an  alternating 
cycle  of  six  pairs,  the  members  of  which  are  free  from  the  stomodaeum 
throughout  their  length ;  (c)  a  variable  number  of  third-cycle  mesenteries, 
alternating  with  the  two  previous  cycles,  but  very  rarely,  if  ever,  with  the 
whole  cycle  of  twelve  pairs  developed  (plate  6,  fig.  34).  Thus  two  cycles 
of  mesenteries  are  always  developed,  and  a  variable  number  belonging  to  a 
third  cycle.  They  may  be  represented  by  the  formula  6,  6,  x,  where  x 
represents  any  number  from  i  to  12. 

Within  each  intermesenterial  space,  whether  an  entocoele  or  an  exocoele, 
an  invagination  of  the  polypal  wall  occurs  which  separates  adjacent  mesen- 
teries from  one  another.  Except  in  the  uppermost  region  these  septal 
invaginations  extend  centrally  further  than  the  mesenteries,  while  in  the 
lower  parts  of  the  polyp  they  extend  so  far  as  to  meet  in  the  middle  and  fuse, 
dividing  the  gastro-coelomic  cavity  into  distinct  chambers,  each  of  which 
contains  a  mesentery  (plate  7,  figs.  38,  39). 


ADULT    COLONY.  25 

The  mesenteries  extend  from  above  downward  for  about  two-tbirds  of 
tbe  total  length  of  the  polyp,  the  younger  outer  pairs  terminating  in  advance 
of  the  older  inner  pairs.  Below  they  become  very  short  transversely,  and 
are  only  slightly  folded  at  their  free  extremity.  Kxcept  in  the  uppermost 
region  of  the  polyp,  their  peripheral  portion  is  everywhere  perforated  by  the 
synapticular  bars.  Further,  they  are  connected  outwardly  with  the  wall  of 
the  polyp  only  in  the  upper  part  of  their  course ;  the  connection  breaks  down 
below,  and  the  organs  are  seen  in  transverse  sections  with  both  their  inner 
and  outer  ends  free.  Thus  at  an  early  stage  the  peripheral  portion  of  the 
mesentery  undergoes  degeneration  or  resorption  much  in  advance  of  the  more 
central  part.  The  mesenteries,  in  fact,  occupy  only  the  middle  and  upper 
part  of  the  gastro-vascular  cavity ;  the  proximal  and  peripheral  regions  are 
practically  destitute  of  them. 

The  mesenterial  mesoglcea  is  usually  narrow,  but  there  is  much  varia- 
tion in  the  different  polyps  as  to  its  thickness  and  hence  that  of  the  mesentery 
as  a  whole.  In  the  upper  region  the  mesoglcea  presents  vertical  folds  for 
the  support  of  the  retractor  muscles  (plate  7,  fig.  41).  Generally  the  fold- 
ings are  diffuse  and  somewhat  complicated,  and  extend  along  the  entire 
face  of  the  mesentery ;  in  other  cases  they  are  stronger  and  more  restricted 
to  the  middle  area.  The  oblique  musculature  is  very  weak  throughout,  but 
occasionally  the  fibrils  can  be  seen  cut  obliquely  in  transverse  sections.  As 
would  be  expected,  no  basilar  or  parieto-basilar  muscles  occur.  The  mesen- 
terial endoderm  is  everywhere  richly  supplied  with  Zooxanthellse,  and  also 
with  clear  gland  cells  and  others  containing  large  granules.  As  described 
in  connection  with  the  endoderm  of  the  column  wall  and  disc  (p.  22),  what 
seems  to  be  an  intercellular  system  of  mucous  spaces  occurs  in  the  deeper 
parts  of  the  epithelium  on  both  mesenterial  face. 

The  arrangement  of  the  mesenteries  may  be  studied  in  more  detail.  As 
represented  on  plate  6,  fig.  34,  and  also  in  the  diagrammatic  figure  on  page  26, 
the  number  of  pairs  within  each  of  the  six  primary  systems  or  -sextants 
varies  from  one  to  three.  The  members  of  a  pair  can  be  easily  distinguished 
by  their  similarity  of  size  and  by  the  retractor  muscles  being  on  the  faces 
turned  towards  each  other.  In  each  of  the  two  ventral  systems  only  one 
pair  of  mesenteries  (11)  is  developed,  while  in  the  middle  and  dorsal  systems 
two  pairs  (11,  iii)  are  present,  with  the  exception  of  the  right  middle  system, 
which  contains  three  pairs.  Where  only  one  pair  occurs,  it  represents  one  of 
the  six  pairs  of  first-cycle  metacnemes  (11),  and  this  cycle  is  complete  in  all  the 
mature  polyps  examined.  Where  two  pairs  are  present,  one  pair  (iii)  is  shorter 
than  the  other  and  is  a  member  of  the  third  cycle.     It  will  be  observed  that 


26  SIDERASTREA    RADIANS. 

in  eacli  sextant  the  third-cycle  pair  (iii)  is  situated  on  tlie  dorsal  aspect 
of  the  second-cycle  pair  (ii).  In  the  right  middle  sextant,  where  two  pairs  of 
third-cycle  mesenteries  occur,  a  pair  is  present  on  both  the  dorsal  and 
ventral  aspects  of  the  longer  pair.  It  is  shown  later  that  the  mesenteries 
of  the  first  and  second  cycles  are  developed  in  a  very  definite  sequence, 
and  such  would  probably  be  the  case  for  the  members  of  the  third  cycle  were 
the  polyps  isolated  and  free  to  develop  normally  all  around.  The  individual 
polyps  in  a  colonial  coral  like  Siderastrea^  however,  are  so  closely  arranged 


Illb 


I 


Fig.  3. — Diagram  of  mesenteries  of  fig.  34,  plate  6  {cf.  figs,  i  and  a,  pp.  12,  13). 

that  their  growth  after  the  first  two  cycles  becomes  largely  influenced  by 
spatial  necessities.  Hence,  in  the  particular  polyp  from  which  plate  6,  fig. 
34,  is  taken,  one  region,  the  right  middle  sextant,  has  progressed  farther 
than  the  others.  Throughout  the  studies  little  constancy  has  been  found  in 
the  order  of  development  of  the  members  of  the  third  cycle.  The  amount  of 
variability  in  the  number  and  disposition  of  the  mesenterial  pairs  is  repeated 
in  the  septa  (p.  48).  In  no  two  polyps,  among  a  dozen  or  so  studied  in 
transverse  sections,  was  the  mesenterial  plan  the  same  for  the  third  cycle ; 
also,  in  none  was  the  full  complement  of  twelve  pairs  present.  The  normal 
sequence  of  the  mesenteries  is  more  fully  described  in  connection  with  the 
development  of  the  polyp. 


ADULT    COLONY.  •  27 

The  radial  length  of  the  mesenteries  varies  greatly  in  passing  trans- 
verse sections  in  review  from  the  oral  to  the  aboral  extremity  of  a  polyp.  In 
an  expanded  polyp  the  complete  mesenteries  extend  from  the  colnmn  wall, 
then  across  the  disc,  and  finally  unite  with  the  stomodaeum,  down  which 
they  pass  ;  the  members  of  the  incomplete  cycles,  on  the  other  hand,  stretch 
from  the  column  wall  only  partly  across  the  disc,  and  terminate  at  a  greater 
or  less  distance  from  the  middle  of  the  polyp.  When  the  level  of  the  corallum 
is  reached,  all  the  mesenteries  begin  to  lose  their  peripheral  connection 
with  the  polypal  wall,  and  in  transverse  sections  hang  freely,  unconnected 
either  peripherally  or  centrally,  except  where  pierced  by  a  synapticulum. 
Even  at  the  level  of  the  stomodaeum  in  partly  retracted  polyps  the  mesen- 
teries are  greatly  shortened  transversely,  so  that  the  peripheral  parts  of 
the  septal  loculi  are  empty  (plate  6,  fig.  34).  Where  the  loculi  are  inter- 
rupted transversely  by  a  synapticulum,  a  mesentery  will  sometimes  stretch 
from  an  inner  chamber  of  the  loculus  to  the  next  chamber,  but  in  no  instance, 
as  shown  on  plate  6,  fig.  34,  is  one  found  to  extend  right  to  the  periphery 
of  an  interseptal  loculus.  Instead  of  this,  most  of  the  peripheral  chambers 
are  empty,  the  mesenteries  having  become  resorbed.  The  organs  have  a 
much  less  radial  or  peripheral  extent  on  plate  7,  fig.  38,  resorption  having 
been  continued  further  than  at  the  level  represented  by  plate  6,  fig.  34. 
Mesenteries  are  wholly  absent  on  plate  7,  fig.  39,  except  in  one  or  two 
isolated  instances.  The  stages  prove  conclusively  that  the  mesenteries 
extend  but  a  very  short  vertical  distance  within  the  region  of  the  synapti- 
cula,  but  are  longer  centrally.  The  same  fact  is  also  well  shown  on  plate  6, 
fig.  36,  representing  the  upper  part  of  a  tangential  section  of  a  polyp. 

The  descriptions  given  by  Bourne  (1887,  1893)  of  the  anatomy  of  the 
synapticulate  coral  Fungia  indicate  similar  degeneration  phenomena  of  the 
mesenteries  within  the  region  of  the  synapticula  in  this  genus.  At  first  both 
the  primary  and  secondary  mesenteries  extend  to  the  very  base  of  the  coral- 
lum, but  afterwards  they  are  confined  to  the  upper  moiety  of  the  calice, 
though  in  Fungia  no  dissepiments  are  formed  which  cut  off  the  polyp 
from  the  basal  plate.  Bourne's  figs.  10,  13,  and  15  in  his  first  paper  show 
that  the  mesenteries  very  rarely  extend  along  the  interseptal  loculi  all  the 
way  from  one  synapticular  perforation  to  another,  but  in  practically  all 
the  canals  long  or  short  remnants  of  the  mesenteries  are  adherent  to  the 
synapticular  wall,  and  serve  for  the  attachment  of  separate  bundles  of  the 
longitudinal  muscles.  In  these  cases  the  middle  part  of  the  mesentery  within 
each  chamber  has  been  resorbed,  leaving  only  its  two  extremities.  In  Sider- 
astrea  there  is  nothing  corresponding  to  the  mesenterial  muscle  bundles  in 


28  SIDERASTREA    RADIANS. 

the  synapticular  region  whicli  Bourne  describes  for  Fungia  and  Fowler  (1888, 
p.  9)  for  Stephanophyllia  jormosissima  ;  muscular  degeneration  proceeds  along 
with  that  of  the  mesentery  in  the  West  Indian  form. 

The  actual  process  of  resorption  of  the  mesenteries,  from  the  periphery 
inwards,  can  be  readily  traced.  It  commences  in  the  peripheral  region  of  the 
polyp  even  as  high  as  the  level  of  the  retracted  stomodseum.  The  part  of 
the  mesentery  undergoing  degeneration  narrows  and  breaks  up  into  separate 
fragments  ;  the  mesogloea  is  often  very  thin  for  some  distance,  and  the  mus- 
cular fibrils  lining  it  form  only  an  irregular,  discontinuous  layer.  Generally 
the  portion  of  the  mesentery  some  little  distance  from  the  peripherally  fixed 
part  is  the  first  to  pass  away,  thus  leaving  an  interval  in  the  actual  continuity 
of  the  organ.  The  endoderm  may  disappear  first,  and  the  naked  mesogloea 
then  extends  a  little  beyond  and  thins  out  (plate  7,  fig.  43),  but  usually  the 
epithelium  persists  the  longest.  Free  fragments  of  the  degenerating  tissue 
are  often  found  in  the  interseptal  chambers,  and  other  fragments  are  occa- 
sionally seen  absorbed  by  the  endoderm  lining  the  chamber. 

Mesenterial  resorption  must  of  course  take  place  proximally  in  all  corals 
which  continue  to  grow  upwards  as  the  lower  part  of  the  polyp  is  cut  off 
below  by  dissepiments.  But  in  Siderastrea  the  peripheral  parts  begin  to 
disappear  first,  and  the  disappearance  as  a  whole  extends  upwards  propor- 
tionally much  further  than  in  most  corals.  Perhaps  this  is  in  some  way 
connected  with  the  presence  of  the  synapticula  in  the  peripheral  region.  In 
other  coral  polyps  the  lower  edge  of  the  mesentery  appears  to  be  resorbed 
more  uniformly. 

The  mesenteries  are  too  small  and  too  completely  inclosed  within  the 
interseptal  chambers  to  permit  of  their  being  dissected  out  and  examined  as 
a  whole,  but  by  means  of  serial  sections  it  can  easily  be  seen  that  the  synap- 
ticula actually  perforate  the  mesenteries.  In  the  first  place,  the  complete 
loculus  shown  on  plate  6,  fig.  33,  reveals  that  the  synapticula  really  per- 
forate the  walls,  and  necessarily  anything  inclosed  within  the  two  lining 
walls  in  the  same  areas.  Then  certain  of  the  synapticular  perforations 
included  in  plate  6,  fig.  34,  show  that  a  mesentery  is  continued  from  each 
wall  bounding  the  perforation,  passing  centrally  on  the  one  hand  and 
peripherally  on  the  other.  On  following  such  a  mesentery  both  upward 
and  downward  in  serial  sections  it  is  found  that  when  the  synapticular 
perforation  disappears,  having  been  followed  through  its  vertical  extent,  the 
two  moities  of  the  mesentery  again  become  continuous.  Clearly  such  condi- 
tions can  result  only  from  the  presence  of  actual  fenestrae  in  the  mesentery. 
Similar  relationships  can  be  also  followed  in  the  series  of  sections  represented 
on  plate  9. 


ADULT    COLONY.  29 

Owing  to  the  early  disappearance  of  the  mesenteries  aborally  and  periph- 
erally the  organs  are,  at  any  one  time,  necessarily  perforated  for  a  very 
restricted  part  of  their  course.  Furthermore,  as  a  result  of  the  presence 
of  these  synapticular  bars,  any  mesentery  must  be  capable  of  less  retraction 
than  in  species  where  the  whole  length  of  the  organ  is  free  to  respond  to  the 
action  of  the  retractor  muscle. 

Desmocytes,  continuous  with  the  mesogloea  of  the  lining  of  the  inter- 
septal  chambers  and  the  attached  mesentery,  are  usually  numerous  around 
the  perforations  of  the  synapticula  (plate  7,  fig.  41,  and  plate  8,  fig.  45). 

MESENTERIAL  FILAMENTS. 

Filaments  occur  on  the  mesenteries  of  all  three  cycles,  though  usually 
they  are  imperfectly  developed  on  the  members  of  the  third.  On  the  incom- 
plete mesenteries  the  organs  commence  a  short  distance  above  the  lower  end 
of  the  stomodseum,  and  are  continued  throughout  the  vertical  extent  of  the 
mesentery  ;  on  the  complete  mesenteries  they  start  from  the  lower  termina- 
tion of  the  stomodaeum,  the  ectoderm  of  the  latter  being  continuous  with  the 
filament.  As  in  all  Madreporaria  yet  described  the  filaments  consist  of  only 
a  median  lobe,  supported  basally  on  an  expanded  mesogloeal  axis  (plate  7, 
fig.  42).  Trilobed  mesenterial  filaments  having  lateral  lobes  supported  on 
a  mesogloeal  axis  and  bearing  ciliated  bands  (Flimmerstreifen),  like  those 
found  in  most  Actiniaria,  are  not  known  to  occur  in  corals. 

The  fully  developed  filaments  of  S.  radians  are  sharply  separated  from 
the  mesenterial  endoderm  by  a  well  marked  constriction  on  each  side.  The 
endoderm  is  usually  slightly  swollen  immediately  behind  the  filament,  but 
rarely  assumes  the  form  of  a  definite  lobe  as  in  many  other  coral  species.  In 
addition,  the  outline  of  the  filament  in  section  varies  somewhat  in  the  case  of 
the  first-cycle  mesenteries.  At  first  it  is  cordate,  and,  histologically,  is  quite 
uniform  all  round;  but  soon  it  becomes  nearly  circular,  and  the  cellular 
constituents  of  the  middle  part  differ  from  those  of  the  lateral  {cf,  plate  7, 
figs.  42,  43). 

The  cellular  constituents  of  the  filaments  are  mostly  ciliated  supporting 
cells,  but  with  these  are  mingled  large,  clear,  and  granular  gland  cells  and 
nematocyst  bearing  cells,  especially  in  the  middle  region  of  the  sections. 
Towards  the  sides  and  posterior  borders  the  cells  diminish  in  height  and 
are  nearly  all  supporting  cells  (plate  7,  fig.  42).  At  least  two  kinds  of 
nematocysts  occur — a  long,  narrow,  thick-walled  form,  similar  to  that  in  the 
tentacular  knobs,  and  a  large,  oval,  thin-walled  form,  with  the  spiral  thread 
strongly  marked  (plate  7,  fig.  44). 


30  SIDERASTREA    RADIANS. 

The  free  end  of  the  mesenteries  and,  consequently,  the  filaments  also  are 
somewhat  exceptional  among  corals  in  the  small  degree  to  which  they  are 
convoluted  in  the  lower  regions.  In  most  cases  they  pass  vertically  in  a 
straight  course  from  their  upper  to  their  lower  extremities  without  any  folding. 
Upon  irritation  the  living  polyps  of  most  corals  are  able  to  extrude,  through 
temporary  apertures  in  the  body-wall,  masses  of  contorted  filaments  along 
with  the  portion  of  the  mesentery  to  which  they  are  attached,  but  this 
phenomenon  has  been  observed  on  only  one  occasion  in  Siderastrea. 

The  question  of  the  ectodermal  or  endodermal  origin  of  the  filaments  is 
alluded  to  in  the  description  of  the  larva. 

SKELETOTROPHIC  TISSUES. 

The  skeletotrophic  or  skeletogenic  tissues  constitute  by  far  the  greatest 
proportion  of  the  soft  parts  of  the  polyps,  including  as  they  do  the  lining  of 
the  calicinal  wall,  septa,  and  columella  throughout.  They  represent  the 
original  flat  basal  disc  of  the  polyp,  which  has  become  greatly  infolded  in 
correspondence  with  the  skeletal  ingrowths.  Both  the  endoderm  and  ecto- 
derm present  certain  structural  peculiarities  compared  with  the  same  layers 
in  the  column  wall  and  disc,  and  in  some  respects  the  calicoblast  layer  of 
S.  radians  differs  from  that  in  most  other  corals. 

The  polypal  wall  lining  the  uppermost  parts  of  the  skeleton  is  extremely 
delicate,  especially  over  the  edges  of  the  septa.  In  sections  the  combined 
ectoderm  and  endoderm  vary  from  0.015  to  0.003  ™^-  i^  thickness,  both  layers 
being  about  equal  (plate  8,  fig.  47).  The  endoderm  is  a  sjaicytium  showing 
no  signs  of  cellular  divisions,  and  the  cytoplasm  has  at  first  few  or  no 
vacuoles,  though  below  they  become  somewhat  numerous.  The  contained 
nuclei  are  large,  round  or  oval,  and  closely  arranged  in  a  very  regular  row. 
Zooxanthellse  also  occur,  their  diameter  being  often  equal  to  the  thickness 
of  the  layer.  Irregular  mucous  spaces,  similar  to  those  described  in  the 
endoderm  (p.  22),  are  also  seen,  except  where  the  layer  is  at  its  thinnest. 

The  mesoglcea,  both  here  and  elsewhere  throughout  the  skeletogenic 
tissues,  is  rarely  distinguishable  as  a  distinct  layer,  but  constitutes  a  mere 
line  of  division  between  the  ectoderm  and  endoderm.  Where  the  mesoglcea  of 
the  mesenteries  is  united  with  the  skeletal  covering  the  calicoblast  ectoderm 
is  nearly  absent,  and  the  mesoglcea  may  then  become  swollen  and  continued 
into  the  well-known  structures  which  have  been  termed  desmocytes  by 
Bourne  (1899).  '^^^  combined  mesenterial  and  skeletotrophic  mesoglcea 
here  broadens  in  a  fan-like  manner,  is  striated,  and  stains  more  deeply  than 
usual  (plate  8,  fig.  45).     Sometimes  the  desmocytes  appear  as  more  distinct 


ADULT  COLONY.  3 1 

triangular  or  wedge-shaped  structures,  with  the  narrow  apex  towards  the 
mesoglcea. 

The  function  of  the  desmocytes  or  desmoidal  processes,  so  closely  asso- 
ciated with  the  mesoglcea,  is  considered  to  be  that  of  attaching  the  soft 
polypal  tissues  to  the  hard  corallum,  and  they  are  always  most  numerous 
along  the  line  of  union  of  the  mesenteries  with  the  basal  wall.  As  the 
mesenteries  in  Siderastrea  are  attached  to  the  skeletogenic  tissues  only 
in  the  uppermost  part  of  the  polyp,  desmocytes  are  developed  somewhat 
sparingly ;  furthermore,  they  are  rarely  found  over  regions  where  the 
mesenteries  are  not  attached.  In  such  areas  a  broad  calicoblast  layer 
usually  intervenes  between  the  mesoglcea  and  the  skeleton.  Where  the 
skeletogenic  tissues,  including  the  mesenteries,  are  perforated  by  the  synap- 
ticula,  desmocytes  are  usually  present  along  the  line  of  attachment  of  the 
mesentery. 

Bourne  (1899)  has  worked  out  in  a  very  masterly  manner  the  origin  of 
the  desmoidal  processes  from  the  single  calicoblast  cells.  These  cells,  which 
are  strictly  the  desmocytes,  later  become  connected  with  the  mesoglcea,  and 
the  structure  takes  on  a  feebly  striated  character.  Although  a  careful  search 
has  been  made  in  sections  of  Siderastrea  prepared  in  various  ways,  I  have 
found  no  certain  stages  in  the  formation  of  the  desmoidal  processes.  Cells 
of  various  forms  occur  within  the  calicoblast  layer  in  regions  where  desmoidal 
processes  are  already  present,  or  may  be  expected,  but  none  which  could  be 
determined  with  certainty  as  desmocytes.  Anyone  familiar  with  the  details 
of  madreporarian  histology  will  appreciate  the  credit  due  to  Bourne  for  work- 
ing out  the  development  of  these  structures  with  such  a  degree  of  complete- 
ness. In  their  fully  formed  condition  they  are  undoubtedly  continuations  of 
the  mesoglcea,  and  it  is  remarkable  that  they  should  have  originated  inde- 
pendently and  later  come  into  fusion  with  the  middle  layer.  Gardiner  states 
(1902,  p.  138)  that  in  the  polyps  of  Flabellum  rubrum  the  first  appearance  of 
any  desmocyte  could  be  seen  in  a  granular  mass  of  protoplasm  against  the 
corallum,  to  which,  from  the  first,  it  seemed  to  be  attached.  Subsequently, 
by  growth  inwards,  it  joins  the  structureless  lamella,  which  may  be  thickened 
so  as  to  meet  it. 

The  actual  calicoblast  layer  of  S.  radians^  like  the  endoderm,  is  at  first 
very  narrow,  and  in  the  growing  areas  of  the  skeleton  shows  no  evidence  of 
cell  limitations.  The  cytoplasm  appears  continuous,  non-granular,  and  with 
or  without  mucous  spaces,  the  whole  staining  a  bright  yellow  in  picric  acid. 
The  nuclei  are  nearly  as  numerous  as  in  the  endoderm,  and  are  large  and 
finely  granular.     The  margin  towards  the  skeleton  is  usually  irregular  in 


32  SIDERASTREA    RADIANS. 

outline,  often  with  many  adhering  particles  which  stain  differently  from  the 
layer  itself  (plate  8,  fig.  47). 

Away  from  the  actual  growing  apices  of  the  skeleton,  both  downward 
and  peripherally,  the  skeleton-lining  tissues  at  once  begin  to  undergo  a 
marked  alteration ;  both  ectoderm  and  endoderm  become  greatly  thickened 
until  they  are  three  or  four  times  their  former  size  (plate  8,  figs.  45-48). 
The  thickened  endoderm  is  reticular,  with  many  oval  nuclei  arranged  in  a 
regular  zone  towards  the  margin ;  Zooxanthellse  are  also  numerous,  and  dis- 
tinct mucous  accumulations  appear. 

Vacuoles  extend  nearly  across  the  layer,  and  usually  appear  clear  and 
perfectly  transparent,  but  when  stained  with  iron  hsematoxylin  they  present 
a  grayish  reticulum.  They  are  evidently  mucous  in  character,  and  would 
seem  to  represent  individual  cells.  In  sections  stained  with  Delafield's 
haematoxylin  and  picric  acid  the  vacuolar  spaces  stand  out  conspicuously, 
the  boundaries  and  reticulum  being  stained  a  dense  blue,  as  with  mucous 
cells  generally,  and  appearing  as  if  enclosed  in  a  yellow  granular  matrix. 
Usually  the  mucous  spaces  do  not  extend  as  far  as  the  outer  margin  of  the 
layer,  and  not  always  to  the  mesogloeal  boundary. 

The  general  cytoplasm  of  the  calicoblast  ectoderm  is  finely  granular 
throughout  and  does  not  stain  readily  except  in  blue-de-Lyon,  which  colors 
the  particles  a  bright  blue.  The  nuclei  are  comparatively  small  and  less 
numerous  than  in  the  upper  part  of  the  layer,  or  in  the  endoderm.  In 
addition  to  the  granular,  matrix-like  cytoplasm,  cells  with  coarse  granules 
occur  here  and  there  which  stain  differently  from  the  other  granules  and 
appear  to  be  altogether  distinct  structural  elements.  In  sections  stained 
with  carmine  and  blue-de-Lyon  they  stand  out  a  conspicuous  red  against  the 
ordinary  blue  granules.  They  rarely  if  ever  extend  wholly  across  the  layer 
and  seem  comparable  with  the  other  coarsely  granular  cells  found  elsewhere 
throughout  the  ectoderm  and  in  the  mesenterial  filaments. 

Scattered  throughout  the  calicoblast  ectoderm  are  also  a  few  small  oval 
nematocysts  with  a  close  spiral  thread.  Sometimes  several  may  occur 
together,  but  usually  they  are  found  singly.  Different  stages  in  their  devel- 
opment also  can  be  observed.  In  the  earlier  stages  they  are  large,  with 
homogeneous  contents,  and  stain  rather  deeply,  whereas  when  mature  they 
stain  feebly,  if  at  all. 

The  presence  of  nematocysts  within  the  calicoblast  layer,  where  appar- 
ently they  can  be  of  no  service  to  the  polyp,  is  somewhat  remarkable.  It  is 
to  be  borne  in  mind,  however,  that  the  calicoblast  layer  is  but  a  modified  part 
of  the  ectoderm  which,  over  the  column  wall  and  oral  disc,  always  bears 


ADULT  COLONY.  33 

stinging  cells ;  and  further,  the  cysts  are  usually  present  in  numbers  at 
the  aboral  extremity  of  the  larva  which  becomes  the  basal  disc  when  the 
larva  settles.  So  far  as  can  be  made  out  from  sections,  without  isolation  by 
maceration,  the  nematocysts  in  the  calicoblast  layer  are  similar  to  those  in 
the  ectoderm  of  the  column  wall.  Bourne  (1899),  in  his  studies  of  the 
calicoblast  layer  of  the  Madreporaria,  also  encountered  oval  bodies  which 
suggested  to  him  degenerate  nematocysts.  The  close  spiral  thread  found 
in  those  of  Siderastrea^  and  their  general  behavior  at  different  stages  towards 
reagents,  leave  no  doubt  as  to  their  true  nature. 

The  structure  of  the  calicoblast  ectoderm,  as  above  given,  differs  some- 
what from  that  usually  met  with  in  corals.  As  described  in  the  papers  of 
Bourne,  Fowler,  and  Gardiner,  the  layer  is  thickest  in  the  regions  of  active 
growth,  and  becomes  very  thin  or  nearly  disappears  elsewhere.  Such  is  also 
its  condition  in  most  of  the  species  of  corals  examined  by  me  (1902,  p.  482). 
It  may  appear  either  as  a  columnar  epithelium  [Madrepord)  or  have  the 
character  of  a  syncytium.  A  well-developed  cellular  tissue,  with  abundant 
protoplasm,  would  be  expected  where  active  formation  of  the  skeleton  is  in 
progress.  Here,  in  Siderastrea^  the  layer  is  thickest  in  those  regions  where 
the  secretory  activity  would  be  considered  to  be  at  a  minimum.  The  granular 
character  of  the  cytoplasm,  the  small  size  of  its  nuclei,  and  the  strong 
vacuolization  would  not,  however,  suggest  that  the  deposition  of  skeletal 
matter  was  actively  proceeding  in  the  lower  regions,  as  compared  with  the 
large  nuclei  and  more  hyaline  cytoplasm  along  the  edges  of  the  septa. 

It  seems  doubtful  as  to  how  far  the  thinness  of  the  endoderm  in  the  upper 
region  is  to  be  associated  with  a  similar  condition  of  the  calicoblast  endoderm. 
Over  the  spines  of  the  columella,  which  are  situated  in  the  lower  regions  of 
the  polypal  cavity,  and  are  probably  in  a  growing  state,  the  two  layers  pre- 
sent a  great  difference  in  this  respect.  The  ectoderm  is  as  thin  as  in  the 
uppermost  growing  parts  of  the  polyp,  while  the  endoderm  is  greatly  thick- 
ened, measuring  as  much  as  0.05  mm.  across,  which  is  nearly  equal  to  the 
combined  thicknesses  of  both  ectoderm  and  endoderm  in  the  lower  regions. 

None  of  the  writers  on  the  anatomy  of  coral  polyps  have  referred  to  the 
marked  alteration  undergone  by  the  skeletotrophic  endoderm  in  passing  from 
the  upper  to  the  lower  parts  of  the  polyp.  It  is,  however,  characteristic  of 
practically  all  West  Indian  corals,  the  contrast  being  usually  greater  than 
that  shown  by  Siderastrea. 

In  his  study  of  the  minute  anatomy  of  the  polyps  of  Flabellum  rubrum^ 
Gardiner  (1902)  found  the  calicoblastic  ectoderm  to  be  in  many  respects 
like  that  of  S.  radians.     The  layer  is  every^vhere  complete  and  persistent, 


34  SIDERASTREA    RADIANS. 

even  in  the  most  roughly  decalcified  specimens,  and  no  definite  cell  limita- 
tions could  be  distinguished ;  but  over  the  greater  part  of  the  corallum  it  is 
an  extremely  thin,  finely  granular  layer,  and  as  the  edges  of  the  septa  are 
approached  the  layer  thickens,  nuclei  become  more  frequent,  and  tend  to 
exhibit  a  definite  network.  Where  secretion  may  be  supposed  to  be  going 
on  most  actively  the  layer  is  more  hyaline,  elsewhere  it  is  granular.  The 
protoplasm  is  much  vacuolated,  as  in  Siderastrea^  except  where  secretory 
activity  prevails. 

Where  decalcification  has  been  carefully  carried  out,  a  layer  of  the 
organic  matrix  within  which  the  skeleton  is  deposited  usually  remains 
behind,  and  is  closely  adherent  to  the  calicoblast  layer,  as  shown  at  the  left 
side  of  plate  8,  fig.  45.  The  matrix  usually  appears  perfectly  homogeneous, 
very  variable  in  width,  and  recalls  the  mesogloea  in  its  behavior  towards 
stains.  Bourne  (1899)  found  such  a  layer  in  many  of  the  forms  examined 
by  him,  and  regards  it  as  a  fine  membrane — "  limiting  membrane" — separat- 
ing the  calicoblasts  from  the  corallum,  and  comparable  with  the  sheath  which 
incloses  the  spicules  of  the  Alcyonaria.  It  is  evidently  a  secretion  of  the 
ectodermal  layer,  and  at  the  apical  points  of  rapidly  growing  corals,  such  as 
Madrepora,  I  have  found  the  deposit  to  be  continuous  throughout  the  whole 
thickness  of  the  corallum,  and  presenting  just  beyond  its  border  the  fibrous 
scale-like  appearance  of  the  early  skeleton  (1902,  p.  484).  It  is  manifest  that 
the  mass  should  be  regarded  as  a  homogeneous,  mesogloea- like  matrix  within 
which  the  minute  calcareous  crystals  forming  the  skeleton  are  laid  down,  to 
be  compared  with  the  matrix  which  Bourne  has  demonstrated  for  the  skeletal 
spicules  of  Alcyonaria.  As  a  continuous  structure  it  early  disappears,  and 
in  the  older  parts  of  the  skeleton  is  represented  merely  by  the  fine  organic 
particles  remaining  after  decalcification.  Only  under  the  most  favorable 
circumstances  is  it  present  as  a  continuous  mass  in  sections  of  the  newest 
formed  parts.  Usually  all  that  remains  of  it  after  decalcification  is  the  deli- 
cate membrane  bordering  the  calicoblast  layer  on  its  skeletal  aspect.  When 
of  sufficient  thickness  to  have  contained  skeletal  fibers,  now  dissolved  away, 
this  membrane  appears  fibrous,  but  immediately  bordering  the  calicoblast 
layer  it  is  homogeneous. 

SEPTAL  INVAGINATIONS,  INTERSEPTAL  LOCULI,  AND  GASTRO-CCELOMIC  CAVITY. 

The  septal  invaginations  {refoulements  septaux  of  Delage  &  H^rouard) 
are  the  vertical,  somewhat  wedge-shaped  infoldings  or  upgrowths  of  the 
basal  wall  which  in  the  living  condition  cover  both  sides  of  the  septa, 
and  have  been  produced  pari  passu  with  them.     Their  presence  results  in 


ADULT  COLONY.  35 

mucli  subdivision  and  complexity  of  the  lower  part  of  tlie  polypal  cavity,  as 
compared  with  that  of  an  actinian,  where  the  basal  disc  retains  its  primitive 
flatness.  In  the  coral  polyp  radial  parts  of  the  basal  disc  are  pushed  vertically 
upwards  into  the  polypal  cavity,  concurrently  with  the  deposition  of  calca- 
reous matter,  so  that  some  portions  of  the  aboral  wall  are  of  the  height  of 
the  septa.  The  earliest  stages  in  the  formation  of  the  basal  infoldings  over 
the  septa  are  shown  on  plate  9,  fig.  53.  In  the  later  processes  of  growth 
the  lower  edge  of  the  column  wall  is  also  carried  upwards  to  about  the  same 
height  as  the  septal  invagination,*  hence  it  results  that  the  basal  wall  is 
represented  by  so  many  vertical  lamellar  infoldings,  the  column  wall  and 
disc  appearing  as  a  mere  superficial  covering  to  these. 

In  transverse  sections  the  invaginations  are  found  to  occur  between 
every  two  mesenteries,  that  is,  they  are  both  entocoelic  and  exoccelic  in 
position,  the  two  series  being  equal  in  number,  the  exoccelic  extending  less 
centrally  than  the  others  (plate  6,  fig.  34).  In  the  upper  region  of  retracted 
polyps  the  invaginations  do  not  extend  as  far  centrally  as  the  primary  mesen- 
teries, though  in  the  stomodseal  region  they  are  continued  beyond  the  secondary 
and  tertiary  mesenteries.  As  the  lower  regions  are  approached  the  foldings 
stretch  centrally  beyond  all  the  mesenteries  until,  in  the  end,  the  walls  of 
adjacent  invaginations  unite  with  one  another  and  thus  completely  inclose 
each  mesentery  in  a  separate  interseptal  loculus,  the  skeletal  deposit  being 
also  continuous  from  the  periphery  to  the  middle  (plate  7,  fig.  39). 

As  shown  in  the  various  figures,  the  septal  invaginations  do  not  remain 
truly  radial.  At  their  inner  extremities  the  exoccelic  members  turn  laterally 
to  unite  with  an  entocoelic  invagination,  and  the  entocoelic  of  the  third  cycle 
are  united  with  the  entocoelic  of  the  second.  In  this  way  continuity  of  the 
invaginated  walls  in  transverse  section  is  broken,  and  the  skeletal  matter 
within  one  invagination  is  likewise  continuous  with  that  in  another.  On  plate 
7,  fig.  39,  it  is  seen  that  some  of  the  septa  are  continued  radially  as  far  as 
the  central  columella,  while  the  others  extend  centrally  only  indirectly  by 
fusing  with  the  first.     As  a  result  of  these  many  fusions  the  cyclic  plan  of 

*  In  many  corals,  e.  g.^  Cladocorti,  Mant'ctna,  the  proximal  margin  of  the  column  wall  is  lower  than 
the  upper  edge  of  the  septa,  and  the  latter  are  generally  united  peripherally  in  the  thecal  wall;  the  septa 
have  grown  upwards  into  the  polypal  cavity,  leaving  the  lower  boundary  of  the  column  wall  behind. 
The  mesenteries  still  remain  attached  to  the  outer  column  wall,  but  internally  they  have  been 
pushed  upwards,  as  it  were,  along  with  the  upward  growth  of  the  thecal  wall.  Owing  to  this,  parts  of 
the  polypal  cavity  and  mesenteries  are  peripheral  or  outside  the  calice  (extracalicular  or  pericalicular) 
and  the  remainder  is  within  the  calice  (intracalicular).  The  portion  of  the  polyp  outside  and  around  the 
calice  is  what  is  known  as  the  "edge-zone"  or  " Randplatte,"  but,  as  noticed  on  p.  9,  there  is  no  edge- 
zone  in  Siderastrea,  the  polypal  cavity  and  mesenteries  being  wholly  intracalicular  when  the  polyps 
are  retracted. 


36  SIDERASTREA    RADIANS. 

the  invaginations  is  not  readily  established ;  in  fact,  it  would  be  almost  impos- 
sible to  determine  it  without  the  assistance  of  the  mesenteries.  In  addition, 
complications  are  produced  by  the  fact  that  the  invaginations  are  perforated 
by  the  synapticula,  and  thus,  in  sections,  their  walls  appear  discontinuous. 

The  gastro-coelomic  cavity  in  Siderastrea  is  very  complicated  in  character 
compared  with  that  of  an  actinian  polyp,  or,  indeed,  with  that  of  many  other 
corals.  A  double  series  of  divisions,  mesenterial  and  septal,  are  to  be  con- 
sidered, except  when  the  polyp  is  fully  expanded  and  the  column  wall  and 
disc  are  raised  wholl}'-  beyond  the  corallum.  In  this  latter  condition  the 
polypal  cavity  is  merely  divided  into  entocoelic  and  exocoelic  chambers  by 
the  mesenteries,  as  in  an  ordinary  actinian  polyp.  Each  chamber  is 
prolonged  into  a  tentacle,  but  there  has  been  found  no  lateral  communica- 
tion between  one  mesenterial  chamber  and  another,  such  as  frequently 
occurs  in  actinians  by  means  of  stomata. 

The  ccelomic  cavities  of  the  many  polyps  making  up  a  colony  are  only 
partly  independent  of  one  another.  At  the  periphery  of  each  polyp  aper- 
tures are  found  between  the  united  edges  of  the  contiguous  column  walls 
and  the  skeletotrophic  tissues,  but  no  apertures  or  canals  elsewhere  connect 
the  different  pol3'ps.  The  communicating  spaces  are  intermesenterial  in 
position,  the  polyps  being  more  or  less  cut  off  from  one  another  mesenterially 
(plate  6,  fig.  35).  The  spaces  divide  the  superficial  column  wall  from  the 
basal  disc.  Hence  in  colonial  polyps  the  column  wall  and  base  are  nowhere 
in  direct  continuity  with  one  another,  except  at  the  free  part  of  the  marginal 
polyps ;  elsewhere  they  are  connected  only  indirectly  by  the  mesenteries  as 
these  pass  from  column  wall  to  basal  disc. 

When  the  level  of  the  corallum  is  reached  the  polypal  cavity  is  encroached 
upon  radially  by  the  septal  invaginations,  and  divided  peripherally  into  inter- 
septal  chambers  or  loculi,  each  of  which  contains  a  single  mesentery.  At 
first  all  the  chambers  communicate  with  one  another  toward  the  middle  of 
the  polyp,  which  is  still  free  from  any  calcareous  deposit  (plate  7,  fig.  38). 
At  the  level  of  the  columella  the  middle  region  of  the  polypal  cavity  is 
encroached  upon,  the  septal  invaginations  extend  from  the  periphery  to  the 
center,  and  the  entire  cavity  is  broken  up  into  distinct  loculi  (plate  7, 
fig-  39)-  These  are  continued  as  far  as  the  aboral  end  of  the  polyp,  where 
each  loculus  terminates  blindly. 

In  addition,  the  interseptal  spaces  of  Siderastrea^  as  of  other  fungids,  are 
encroached  upon  concentrically  by  the  synapticula  which  stretch  across  from 
septum  to  septum,  and  in  sections  give  the  discontinuous  canal-like  character 
to  the  polypal  cavity  seen  towards  the  periphery  of  figs.  38  and  39,  on  plate  7. 


ADULT   COLONY.  37 

In  many  corals  tlie  endodenn  in  tHe  lower  regions  becomes  thickened  to  such 
a  degree  as  to  occupy  the  entire  interseptal  loculi,  but  in  S.  radians  a  narrow 
space  remains  as  far  as  the  aboral  termination. 

GONADS. 

Among  all  the  polyps  which  have  been  examined  anatomically  ova  only 
have  been  found  to  occur.  The  polyps  studied  were  taken  from  colonies 
other  than  those  which  extruded  larvae.  The  gonads  are  situated  below  the 
stomodseal  region,  only  one  ripe  ovum  as  a  rule  being  present  on  each  mesentery 
(plate  7,  fig.  43).  They  occur  about  the  middle  of  the  transverse  length  of 
the  mesentery,  and  are  found  on  the  members  of  both  first  and  second  orders. 
The  ova  are  usually  longer  in  diameter  along  the  radial  axis,  and  are  indented 
where  the  septal  granules  have  intruded  upon  the  narrow  interseptal  chambers. 
Along  with  the  single  large  egg  may  be  two  or  three  developing  ova  without 
yolk,  but  the  conditions  are  not  favorable  for  studying  their  origin  from  the 
ordinary  endodermal  epithelial  cells. 

The  absence  of  male  sexual  elements  from  the  examples  studied  can  by  no 
means  be  taken  as  indicating  a  dioecious  character  of  the  species.     Wherever  « 

thus  far  in  West  Indian  corals  sexual  differentiation  is  suggested  the  gonads 
have  been  found  to  be  female,  but  where  the  ova  are  best  developed  sperm  aria 
have  been  found  to  accompany  them ;  spermaria  have  never  been  found  alone 
in  any  polyp.  Hence  there  is  much  which  suggests  that  coral  polyps  are  pro- 
togynous.  Mr.  Stanley  Gardiner,  however,  has  recently  discussed  the  subject 
of  protandry  in  corals  (Proc.  Camb.  Phil.  Soc,  XI,  1902).  From  an  investi- 
gation of  a  large  number  of  developmental  stages  in  Flabellum  rubrum  he  has 
been  able  to  show  that  in  this  form  spermaria  arise  first  on  the  mesenteries  and 
that  ova  appear  later,  when  the  production  of  sperm  acini  ceases.  "  The  ova 
grow  enormously,  with  the  final  result  that  the  mass  becomes  entirely  female, 
consisting  of  usually  2  or  3  large  ova,  flattened  on  their  sides  against  one  another 
and  occupying  the  whole  area  of  the  former  testes."  Protandry  is  thus  clearly 
established  in  Flabellum  rubrum.  The  claim  for  protogyny  in  the  West  Indian 
corals  hitherto  studied  rests  upon  the  fact  that  spermaria  have  never  been 
found  alone,  but  always  in  association  with  large  numbers  of  ova;  on  the  V 

other  hand,  many  polyps  have  been  found  with  ova  alone,  often  few  in  number,  _  ^ 
as  if  sexual  maturity  were  but  beginning. 

Were  a  general  protandry  to  be  assumed,  we  should  have  to  suppose 
that  all  the  polyps  containing  only  ova  had  passed  beyond  their  male 
period  and  extruded  all  their  spermatozoa,  becoming  wholly  female.  Prob- 
ably, as  in  other  groups  of  animals  and  plants,  no  hard  and  fast  rule  is 


38  SIDERASTREA    RADIANS. 

followed  by  tlie  different  species  of  corals ;  some  will  be  protandrous  wbile 
others  will  be  protogynous. 

It  is  clear  from  the  evidence  adduced  that  a  dioecious  character  can  not 
be  assumed  from  the  presence  of  only  ova  or  sperm  aria  until  the  entire 
developmental  history  of  the  species  has  been  followed ;  a  polyp  having  at 
one  time  only  one  kind  of  sexual  cells  may  later  show  both  kinds. 

CORALLUM. 

Freshly  macerated  coralla  are  generally  white  or  yellowish  in  color,  but 
frequently  they  are  somewhat  green,  either  as  a  whole  or  in  patches.  On 
breaking  a  green  corallum  across  the  coloration  is  found  to  extend  some 
little  distance  below  the  surface,  and,  after  decalcification,  there  remains 
behind  a  green  mass  of  fluffy  texture,  which,  on  examination  under  the 
microscope,  is  found  to  consist  of  filamentous  algas.  Bven  where  the  skeleton 
presents  no  superficial  green  coloration,  a  filamentous  mass  remains  behind 
on  decalcification,  and  chlorophyll  may  be  present  in  small  quantities,  or  the 
filaments  may  be  dead  and  altogether  devoid  of  protoplasmic  contents.  Under 
the  microscope  fragments  of  the  skeleton  frequently  show  fibers  crossing  the 
interseptal  spaces  or  meandering  over  the  surface,  and  sections  of  the  corallum 
are  sometimes  perforated  by  similar  filaments  (plate  ii,  fig.  66).  Thus 
the  algae  actually  bore  their  way  into  the  skeleton  as  well  as  spread  over  the 
surface.     They  are  particularly  abundant  in  decaying  masses  of  coral. 

The  coral-boring  algae  liberated  by  decalcification  are  represented  by 
both  a  non-septate  and  a  septate  form,  the  filaments  of  both  varying  greatly 
in  size.  Associated  with  the  algal  filaments  are  clusters  of  a  Leptothrix-VCsjt 
bacterium ;  but  whether  its  threads  actually  perforate  the  skeletal  matter,  or 
merely  grow  over  its  surface,  living  upon  the  traces  of  organic  matter,  seems 
uncertain.  In  a  former  paper  I  have  pointed  out  the  almost  universal 
presence  of  boring  algae  in  corals,  and  their  importance  in  disintegration 
(Bull.  Amer.  Mus.  Nat.  Hist,  xvi,  1902). 

In  addition  to  the  parasitic  algae,  organisms  such  as  cirripedes,  molluscs, 
or  tubiculous  worms  usually  occur,  either  inclosed  within  the  skeleton  or 
adherent  to  its  surface.  These  associated  organisms  are  particularly  numer- 
ous on  specimens  obtained  from  muddy  shores  as  compared  with  those  from 
the  clearer  waters  of  the  reefs,  and  often  interfere  with  the  regularity  of  form 
of  the  corallites  of  the  colony.  The  cavities  resulting  from  the  activity  of 
the  boring  molluscs  weaken  the  skeleton  greatly,  so  that  the  colonies  can  be 
readily  broken  into  fragments. 

Numbers  of  the  parasitic  cirripede  Pyrgoma  sometimes  occur,  exhibiting 


ADULT  COLONY.  39 

all  stages  in  tlie  growtli  of  its  protective  shell.  The  older  specimens  are 
completely  inclosed  and  fused  within  the  corallum,  only  the  mouth  of  the 
shell  being  exposed  at  the  surface  of  the  colony.  Its  outer  surface  is  covered 
by  an  irregular  growth  of  the  coral  skeleton,  and,  being  raised  a  little  above 
the  general  surface  of  the  corallum,  gives  a  marked  irregularity  to  the  colony 
as  a  whole.  The  external  ridges  on  the  shell  of  the  parasite  have  a  curious 
resemblance  to  the  septa  of  the  coral,  and  in  the  later  growths  it  is  somewhat 
difficult  to  distinguish  one  from  the  other. 

On  certain  macerated  colonies  many  early  stages  were  obtained  showing 
the  manner  of  attachment  of  the  cirripede  within  the  living  calice.  At  first 
the  presence  of  the  small  shell  of  the  crustacean  has  produced  no  modifica- 
tion of  the  coral ;  it  is  simply  adherent  to  a  few  of  the  septa.  But  later  the 
lower  ridges  on  the  surface  of  the  cirripede,  which  closely  simulate  septa, 
became  fused  and  partly  overgrown  with  the  septa,  thus  causing  the  oblitera- 
tion of  the  middle  of  the  calice.  In  every  case  the  intruder  has  fixed  itself 
within  a  calicinal  cavity,  never  on  the  ridge  connecting  two  calices.  Appar- 
ently the  cirripede  larva  enters  the  polypal  cavity  and  then  bores  through 
the  living  tissues  to  the  corallum,  and  in  the  process  of  growth  the  skeletons 
of  the  two  become  fused.  Afterwards  the  two  organisms  continue  growing 
together,  the  coral  skeleton  almost  wholly  inclosing  the  crustacean ;  the  latter 
has  no  boring  action.  In  the  later  stages  the  presence  of  the  Pyrgoma 
results  in  the  production  around  it  of  imperfectly  developed  polyps. 

On  breaking  through  a  colony  it  is  frequently  found  that  the  corallum 
is  made  up  of  growths  of  different  periods,  arranged  upon  one  another  in 
irregular  layers,  instead  of  a  continuous  increase  in  thickness  from  the  middle 
to  the  circumference.  An  old  corallar  surface  has  become  dead  and  partly 
corroded,  and  then  a  later  skeletal  deposit  has  been  formed  upon  this,  and  so 
on  for  several  successive  stages,  the  vertical  axis  of  the  corallites  of  the  two 
or  more  periods  of  growth  usually  corresponding.  The  secondary  origin  of 
polyps  in  old  calices  has  been  already  described  (p.  1 7) .  The  growth  of  the 
corallum  seems  to  be  more  continuous  in  blocks  of  the  larger  S.  siderea  than 
in  S.  radians. 

The  actual  degree  of  calcification  of  the  corallum,  if  one  may  thus  express 
it,  varies  much  in  different  colonies,  and  also  in  different  regions  of  the  same 
colony.  In  some  examples  the  septa  are  comparatively  thick,  leaving  but 
narrow  interseptal  spaces,  and  the  calices  are  shallower  than  usual,  the 
columella  being  solid  and  prominent.  In  others  the  septa  are  thinner,  the 
interspaces  correspondingly  wider,  the  calices  deep,  and  the  columella  either 
absent  as  a  definite  projection  or  represented  by  one,  two,  or  three  prominent 


40  SIDERASTREA     RADIANS. 

granules  on  the  floor  of  the  calice.  Probably  these  differenc'es  are  determined 
by  the  varying  conditions  of  growth  of  the  polyps,  whether  rapid  or  slow. 
The  growing  marginal  corallites  are  always  of  the  less  calcified  variety. 
Such  possible  variations  should  be  borne  in  mind,  for  if  extreme  coralla  only 
were  available  for  study  they  might  be  almost  regarded  as  distinct  species. 

Examination  of  the  surface  of  the  corallum,  and  also  of  sections,  shows 
that  the  individual  calices  of  a  colony  are  separated  from  one  another  only  by 
fusion  of  the  peripheral  edges  of  the  septa.  There  is  no  thecal  formation 
distinct  from  that  of  the  septa.  The  fusion  is  complete  and  continuous,  how- 
ever, and  results  in  the  entire  lateral  separation  of  one  calicinal  cavity  from 
another.  This  is  clearly  shown  in  the  various  figures  of  the  corallum  on 
plate  lo ;  and,  as  further  proof,  it  may  be  recalled  that  on  decalcification  the 
tissues  of  the  different  polyps  so  far  as  they  are  inclosed  within  the  corallum 
are  wholly  cut  off  from  one  another. 

The  septa  of  adjacent  corallites  correspond  peripherally,  end  to  end,  or 
they  may  alternate ;  and  between  these  two  extremes  are  all  intermediate 
conditions  (plate  lo,  figs.  62,  64).  At  the  periphery  the  thickness  of  the  septa  is 
usually  greater  than  the  width  of  the  interseptal  spaces,  so  that  where  the 
septa  of  adjacent  corallites  alternate  it  is  clear  that  a  distinct  wall  of  separation 
is  produced  between  one  calice  and  another ;  but  where  adjacent  septa  are  end 
to  end  a  shallow  interval  may  remain  on  each  side  of  the  two  continuous 
septa,  and  the  septa  are  then  feebly  exsert. 

The  exsert  condition  of  the  septa  is  most  marked  toward  the  margin  of 
colonies,  that  is,  in  the  region  of  new  growth.  Elsewhere  the  intervals  are 
generally  filled  by  calcareous  deposits,  so  as  to  bring  the  calicinal  edge  to 
the  same  level  as  the  septa.  The  usual  superficial  appearance  is  as  if  each 
septum  became  bifurcated  at  its  outer  edge  and  then  each  half  united  with 
a  similar  half  of  two  adjacent  septa  belonging  to  a  contiguous  corallite. 
The  result  is  a  very  narrow  zig-zag  partition  wall  between  adjacent  corallites. 
Usually  the  calicinal  boundary  as  a  whole  reaches  the  same  height  as 
the  septa,  but  occasionally  it  is  depressed,  and  the  septal  edges  are  inclined 
towards  it. 

HISTOLOGY. 

The  different  parts  of  the  corallite  will  be  now  described,  but  first  the 
microscopic  structure  of  the  skeleton  must  be  briefly  considered.  In  connec- 
tion therewith  the  writer  has  to  express  his  indebtedness  to  the.  valuable 
work  of  Miss  Maria  M.  Ogilvie  (Mrs.  Ogilvie-Gordon)  who,  in  her  paper, 
Microscopic  and  Systematic  Study  of  Madreporarian  Types  of  Corals  (1897), 
has  done  more  than  any  other  investigator  to  further  the  histological  study 


ADULT  COLONY.  41 

of  tlie  madreporarian  skeleton,  following  along  lines  initiated  by  Pratz  (Palse- 
ontographica,  vol.  xxix).  Mr.  T.  Wayland  Vaughan  has  given  a  resume 
of  the  subject  in  the  introduction  to  his  paper  on  The  Kocene  and  Lower 
Oligocene  Coral  Faunas  of  the  United  States  (1900). 

When  the  larger  septa  of  Siderastrea  are  examined  sideways,  under  a 
low  magnification,  they  present  the  appearance  of  the  three  septa  shown  on 
plate  10,  fig.  63.  The  peripheral  vertical  boundary  of  each  septum  consists 
of  a  narrow,  continuous,  nearly  straight  ridge,  which  is  the  broken  surface 
of  the  thecal  wall,  really  a  part  of  the  adjacent  septum.  Next  this  is  the 
synapticular  area,  the  synapticula  being  arranged  rather  closely  in  several 
vertical  rows.  The  surface  as  a  whole  is  striate,  becoming  distinctly  ridged 
and  grooved  as  the  margin  is  approached.  The  septum  terminates  centrally 
and  above  in  a  strongly  serrated  margin,  the  teeth  being  almost  conical  in 
shape,  continuous  with  the  ridges,  and  varying  but  little  in  size.  Minute 
granules,  usually  terminating  in  one  or  more  sharp  points,  occur  over  the 
whole  surface,  including  the  synapticular  area  and  even  the  marginal  spines. 

On  septa  of  the  second  and  third  orders  may  also  be  seen  a  vertical  row 
of  synapticulum-like  structures  near  their  inner  border.  They  are  shown  on 
the  right  (central)  margin  of  the  left  septum  on  plate  10,  fig.  63,  and  represent 
the  line  of  fusion  or  coalescence  of  the  next  adjacent  septum  in  the  series — 
the  third  with  the  second,  and  the  fourth  with  the  third.  They  do  not  occur 
on  septa  of  the  first  order,  as  none  of  the  other  septa  fuse  with  these  {cf.  plate 
10,  fig.  64).  The  disconnected  character  of  the  bodies  serves  to  demonstrate 
that  the  central  union  of  one  septum  with  another  by  the  internal  margin  is 
not  continuous,  but  interrupted  in  character.  It  is  best  regarded  as  taking 
place  along  the  spinous  or  toothed  edge  of  the  smaller  septum,  in  the  same 
way  as  the  complete  septa  of  the  first  and  second  orders  fuse  in  an  interrupted 
manner  with  the  columella  by  their  spinous  margins.  In  Siderastrea  Miss 
Ogilvie  (1897,  p.  179)  regards  the  direct  lateral  coalescence  of  the  septal 
surfaces  belonging  to  different  septal  cycles  as  homologous  with  the  synap- 
ticular union  of  septa,  but  it  may  be  pointed  out  that  the  fusion  is  altogether 
on  the  part  of  one  of  the  septa,  not  a  simultaneous  growth  from  two  adjacent 
septa,  as  in  synapticula  proper.  The  interrupted  coalescence  of  the  septa, 
both  with  one  another  and  with  the  columella,  necessarily  leads  to  the  pro- 
duction of  foramina  near  their  inner  edge,  some  of  which  are  shown  to  the 
left  in  plate  10,  fig.  63. 

The  septal  striae  are  directed  upward  and  inward  in  a  half  fan-shaped 
manner,  starting  from  the  vertical  thecal  wall.  Those  on  adjacent  septa 
belonging  to  contiguous  calices  radiate  in  an  opposite  manner,  each  towards 


42  SIDERASTREA    RADIANS. 

the  center  of  its  own  calice  (plate  lo,  fig.  63).  The  thecal  ridge  is  thus  the 
area  of  divergence  of  the  striae  of  adjacent  septa.  It  is  further  found  that 
the  striae  on  opposite  sides  of  the  same  septum  correspond  with  one  another, 
as  can  be  seen  by  looking  edgewise  at  the  inner  margin  of  an  individual 
septum  ;  hence  opposite  striae  terminate  in  one  serra.  The  synapticula  and 
granules  are  set  mainly  along  the  ridges  of  the  septal  face,  but  the  former 
may  be  so  broad  as  to  extend  over  more  than  one  stria. 

In  many  corals  lighter  lines  run  transversely  across  the  striae,  parallel 
to  the  dentate  septal  edge,  and  mark  successive  septal  margins,  corresponding 
to  distinct  periods  of  growth  in  the  life  of  the  polyp.  The  part  between  two 
successive  curves  represents  the  addition  made  to  the  septum  during  a  single 
period  of  polypal  growth,  and  is  called  by  Miss  Ogilvie  (p.  no)  "the  growth- 
segment  of  the  septum  "  or  "  a  septal  segment."  The  septa  of  Siderastrea 
very  rarely  exhibit  such  lighter  cross  lines ;  by  Miss  Ogilvie  the  synapticula 
are  considered  an  essential  part  of  every  growth  segment  in  this  genus. 

A  magnified  transverse  section  through  a  portion  of  the  corallum  shows 
that  the  septa,  columella,  and  synapticula  are  built  up  of  the  same  structural 
elements  or  units  (plate  10,  fig.  65).  These  appear  in  sections  as  so  many 
distinctly  round  or  polygonal  bodies,  fused  together  into  a  compact  continuous 
whole.  Bach  unit  of  structure  under  low  power  reveals  lighter  and  darker 
parts  arranged  in  a  concentric  manner,  and  under  high  power  is  found  to  be 
made  up  of  radiating  calcareous  fibers  (plate  11,  fig.  66).  The  fibers  seem 
to  start  some  little  distance  from  a  center,  which  is  either  clear  or  dark,  and 
practically  homogeneous  in  structure ;  further,  they  are  seen  to  be  arranged 
in  close  concentric  lamellae,  and  the  boundary  between  each  two  layers  is 
often  indicated  by  dark,  finely  granular  particles,  similar  to  those  at  the 
centers  of  calcification. 

Employing  the  terminology  of  Miss  Ogilvie,  each  structural  unit  is  a 
trabecular  here  seen  in  transverse  section.  The  radiating  bundles  of  fibers 
zx^  fascicles r  the  light  or  dark  centers  from  which  the  fibers  radiate  are  centers 
of  calcification r  and  the  concentric  layers  qxq.  growth  lamellce. 

In  transverse  sections  of  the  septa  of  Siderastrea  the  centers  of  calcifica- 
tion are  some  distance  from  one  another,  and  each  bears  radiating  calcareous 
fibers  all  round ;  but  in  the  majority  of  corals  the  centers  are  closer,  and 
often  appear  in  sections  of  the  septa  as  a  more  or  less  continuous  dark  line 
or  band  along  the  septal  axis — the  dark  line  of  calcification.  In  this  case  the 
separate  fascicles  or  bundles  of  fibers  are  more  distinct,  and  arranged  in  a 
feather-like  manner  along  each  side  of  the  dark  line  or  layer. 

Much  discussion  has  taken  place  with  regard  to  the  nature  of  the  center 


ADULT   COLONY.  43 

or  dark  line  of  calcification.  Writers  on  fossil  corals  particularly  have  devoted 
their  attention  to  the  subject.  For  a  complete  account  the  reader  is  referred 
to  various  sections  of  Miss  Ogilvie's  work.  By  some  authors  the  median 
dark  layer  has  been  interpreted  as  2i primary  septum^  upon  the  faces  of  which 
the  lighter  layers,  then  termed  stereoplasm^  have  been  deposited,  the  two 
formations  making  up  the  septum  proper.  Miss  Ogilvie  considers  that  the 
dark  appearance  of  the  centers  results  from  the  presence  of  the  carbonized 
residue  of  the  originally  unchanged  parts  of  the  calicoblasts  within  which 
she  considers  the  madreporarian  skeleton  to  be  formed.  It  must  be  stated, 
however,  that  the  dark  appearance  is  only  seen  when  sections  are  viewed  by 
transmitted  light.  With  reflected  light  the  middle  region  appears  lighter 
than  the  rest  of  the  septum,  and  thus  can  scarcely  be  occupied  by  black 
organic  matter. 

Undoubtedly  the  middle  line  of  the  septum  is  often  of  a  different  struc- 
tural nature  from  the  two  sides,  and  varies  much  in  character  in  different 
species.  In  Paleozoic  corals  I  have  occasionally  found  the  middle  part  to  be 
dissolved  away,  making  the  individual  septum  appear  as  if  formed  of  two 
distinct  lamellae,  more  or  less  completely  separated.  Furthermore,  in  the 
very  young  septa  represented  on  plate  ii,  fig.  70,  the  center  of  each  trabecula 
is  clearly  seen,  under  high  power,  to  be  devoid  of  any  inorganic  matter.  In 
the  rather  thick  section  one  can  focus  down  each  center  for  some  distance, 
which  would  certainly  not  be  the  case  were  it  occupied  by  a  calcareous 
deposit.  I  conceive  that  the  so-called  center  of  calcification  is  really  the 
organic  center  or  axis  around  which  the  skeletal  matter  is  deposited  in  a 
radiating  or  feather-like  manner,  and  that,  at  an  early  stage  in  the  living, 
growing  skeleton,  the  center  is  occupied  by  the  mesogloea-like  matrix  within 
which  it  has  been  shown  that  the  calcareous  fibro-crystals  are  deposited  (p.  34). 
The  organic  matrix  of  the  centers  is  probably  in  all  cases  impregnated 
later  with  calcareous  matter,  differing  in  character  and  crystallographic 
orientation  from  the  true  skeleton  (stereoplasm),  as  shown  by  its  different 
appearance  in  sections  and  different  solubility.  Bourne  has  remarked  (1899, 
p.  ^39) :  "  I  suspect  that  the  dark  *  centers  of  calcification '  will  be  found  to  be 
the  expression  of  a  core  of  organic  filaments,  just  as  the  central  dark  line  in 
the  Alcyonarian  spicule  is  the  expression  of  the  central  core  of  threads." 
Results  published  since  Bourne's  paper  seem  to  confirm  his  conjecture,  only 
the  core  is  probably  homogeneous,  and  later  becomes  impregnated  with  inor- 
ganic matter,  probably  from  solution,  not  from  calicoblastic  activity. 

The  centers  of  calcification  in  5.  radians  are  distant  from  one  another 
on  an  average  about  0.125  mm.  and  the  trabeculae  show  a  corresponding 


44  SIDERASTREA    RADIANS. 

diameter.  The  latter  are  very  variable  in  size  in  the  columella  (plate  lo, 
fig.  65).  Sometimes  two  centers  of  calcification,  diverging  from  one  another 
in  the  median  plane,  are  present  in  one  concentric  system,  and  the  trabecula 
then  appears  double  and  is  elongated  in  outline.  Fusion  between  adjacent 
trabeculse  is  usually  indicated  by  a  narrow,  dark  line. 

A  synapticulum  may  have  either  a  separate  center  of  calcification  (true 
synapticulum)  or  be  formed  from  the  fascicles  of  trabeculae  in  adjacent  septa 
which  extend  beyond  the  septa  until  they  meet  in  the  middle  of  the  inter- 
septal  space  (false  synapticulum,  p.  52).  Bundles  of  fibers  or  fascicles  from 
individual  centers  also  pass  into  the  granulations  over  each  face  of  the  septa. 

The  growth  lamellae  in  the  individual  trabeculae  are  not  always  clearly 
indicated,  but  sometimes  they  come  out  with  remarkable  distinctness,  having 
a  black,  finely  granular  margin  (plate  11,  figs.  66,  68).  Around  the  centers 
of  calcification  they  are  strictly  concentric,  but  towards  the  edges  of  the  septa 
they  more  nearly  follow  the  septal  outline.  In  transverse  sections  they  are 
distant  from  one  another  about  0.0025  ^™-  '^^^  black,  finely  granular 
margin  is  probably  organic  residue  of  the  same  character  as  that  at  the 
centers  of  the  trabeculae. 

A  thin  vertical  section  of  part  of  two  contiguous  septa  is  represented  on 
plate  II,  fig.  67,  and  displays  a  series  of  elongated  bands.  One  of  these,  the 
continuous  median  band,  represents  the  thecal  wall,  and  from  it  the  con- 
stituent parts  of  each  septum  diverge.  The  darker  circular  or  oval  areas  also 
seen  in  the  section  are  the  transversely  cut  surfaces  of  synapticula,  and  each 
originates  in  connection  with  a  band. 

The  broad  radiating  bands  are  the  septal  trabeculae  as  they  appear  in 
longitudinal  section.  Where  the  section  passes  through  the  middle  of  a 
trabecula  a  row  of  close  centers  of  calcification  is  revealed,  and  fascicles  of 
fibers  pass  from  them  obliquely  upwards  on  both  sides.  Peripherally  the 
fibers  of  adjacent  trabeculae  meet  all  the  way  in  a  distinct  thin  line,  so  that  a 
compact  septum  is  the  result.  Fresh  trabeculae  are  seen  to  be  inserted  at 
intervals  between  the  old.  The  trabeculae  are  continued  upward  to  the  septal 
margin  and  there  constitute  the  terminal  teeth,  each  tooth  corresponding  with 
a  single  trabecula,  but  the  preparation  from  which  plate  11,  fig.  Sy,  is  taken 
was  not  sufficiently  wide  to  show  the  teeth.  They  vary  but  slightly  in  width, 
being  about  0.125  ^^^-  across  towards  the  margin  of  the  septum.  From  the 
inclined  arrangement  of  the  trabeculae  it  is  obvious  that  a  transverse  section 
of  the  septum  will  cut  only  the  more  peripheral  at  right  angles ;  the  others 
will  be  represented  in  varying  degrees  of  obliquity. 

Each  separate  dark  area  along  the  middle  of  a  trabecula  represents  a 


ADULT  COLONY.  45 

center  of  calcification,  and  from  it  fascicles  of  fibers  radiate  on  both  sides,  the 
black,  finely  granular  matter  extending  but  a  very  short  distance.  Bach 
center  of  calcification,  therefore,  represents  a  single  growth  period  in  the 
upward  progress  of  the  septum.  They  are  distant  from  one  another  about 
0.013  mm.  A  trabecula  is  thus  a  vertical  series  of  fibrous  groups,  each  group 
of  fibers  being  deposited  around  a  distinct  axis  of  calcification.  A  trabecular 
part  is  the  name  given  to  the  separate  groups  of  fibers  formed  by  the  fasci- 
cles at  one  center  of  calcification.  Bach  stria  seen  under  low  power  on  the 
surface  of  a  septum  represents  a  single  trabecula,  and  the  tooth  in  which  an 
opposite  pair  of  striae  terminates  at  the  edge  of  the  septum  is  the  growing 
apex  or  organic  center  of  the  trabecula.  Around  this  organic  center  new 
calcareous  matter  is  constantly  being  added  by  the  activity  of  the  calicoblasts. 

WALL  OR  THECA. 

The  wall  or  theca  of  a  simple  coral  or  of  a  corallite  is  defined  by  Vaughan 
(1900,  p.  48)  as  "  that  part  of  the  skeleton  that  cuts  off  more  or  less  com- 
pletely the  interseptal  loculus  from  peripheral  communication  with  the 
outside."  In  Siderastrea  the  thecal  wall  at  first  appears  indistinct,  as  if 
represented  only  by  the  united  peripheral  edges  of  the  septa  of  adjacent 
corallites.  Some  of  the  septa  of  contiguous  calices  are  in  the  same  straight 
line,  in  which  case  it  is  difficult  to  distinguish  just  where  the  calicinal 
boundary  comes ;  in  other  places  a  septum  of  one  corallite  corresponds  with 
an  interseptal  loculus  of  the  other,  when  the  distinction  between  the  two 
calices  is  quite  clear.  Usually  it  appears  as  if  the  septa  were  forked 
peripherally,  and  the  two  limbs  of  one  septum  unite  in  a  zigzag  manner 
with  two  adjacent  septa  in  the  next  calice. 

The  nature  of  the  wall  is  best  seen  when  a  corallum  is  fractured  verti- 
cally, and  adjacent  corallites  can  be  viewed  lengthways.  As  indicated  on 
plate  10,  fig.  63,  the  theca  is  then  represented  by  a  narrow,  vertical,  continuous 
ridge,  which  completely  separates  the  interseptal  loculus  of  one  calice  from 
that  of  the  other.  Further,  in  the  section  shown  on  plate  11,  fig.  6'j^  this 
vertical  ridge  is  found  to  consist  of  a  distinct  trabecula,  and  from  it  the 
trabeculae  of  adjacent  septa  diverge. 

Miss  Ogilvie  (1897,  p.  180)  has  given  a  figure  of  Siderastrea  (sp.  ?) 
somewhat  similar  to  that  of  plate  10,  fig.  63,  but  the  thecal  wall  (pseudotheca) 
is  there  represented  as  a  discontinuous  ridge,  as  if  comparable  with  a  row  of 
synapticula,  and  in  such  a  way  as  would  permit  of  communication  between 
one  calice  and  another.  This,  however,  is  not  its  condition  in  S.  radians. 
In  all  instances  I  have  found  the  wall  to  be  continuous,  and  the  fact  that 


46  SIDERASTREA    RADIANS. 

when  a  colony  is  decalcified  adjacent  polyps  are  found  altogether  cut  off  from 
one  another  would  prove  that  their  cavities  are  not  in  lateral  communication, 
as  would  be  the  case  were  the  boundary  wall  interrupted. 

The  to^rms  pseudotheca  and  eutheca  of  von  Heider  (1886)  are  well  known 
to  coral  students,  the  former  referring  to  a  theca  produced  by  the  septa 
becoming  so  much  thickened  peripherally  that  they  fuse  together,  while  the 
latter,  is  applied  to  a  theca  formed  from  separate  trabeculas  between  the 
peripheral  ends  of  the  septa.  Vaughan  (1900,  pp.  48-52)  has  considered 
very  fully  the  value  to  be  applied  to  these  distinctions,  and  comes  to  the 
conclusion  that  "  From  the  great  variation  not  only  in  the  same  species,  but 
in  a  section  of  a  single  corallite,  no  special  systematic  importance  can  be 
attached  to  the  theca  being  of  the  so-called  true  or  false  variety.  True  theca 
marks  the  Anlagen  of  new  septa,  as  von  Koch  and  Bourne  have  shown,  or 
occurs  in  calices  where  the  septa  are  distant  from  one  another  and  their  outer 
ends  are  not  sufficiently  thickened  to  effect  peripheral  fusion." 

The  theca  of  Siderastrea  is  described  by  Miss  Ogilvie  as  a  pseudotheca, 
and,  as  above  shown,  it  is  certainly  formed  at  the  peripheral  extremities  of  the 
septa.  On  the  other  hand  one,  or  rarely  more,  distinct  centers  of  calcification 
occur  laterally  in  the  interval  between  one  septum  and  another,  though  not 
to  be  distinguished  from  those  of  the  septa  except  by  being  inclined  to  them  at 
an  angle.  In  her  figs.  43<3;,  43<5  (p.  181),  Miss  Ogilvie  represents  such  centers 
of  calcification,  but  regards  them  as  synapticular.  Although  presenting  the 
same  appearance  in  transverse  sections,  plate  10,  fig.  63,  and  plate  11,  fig.  67, 
show  that  this  is  not  their  true  character ;  rather,  the  thecal  trabeculse  are 
continuous  vertical  partitions.  According  to  the  accepted  definitions  it  would 
seem  that  the  theca  of  Siderastrea  should  be  regarded  as  a  true  theca,  having 
a  separate  trabecula  between  the  ends  of  adjacent  septa  from  which  the  tra- 
beculae  of  the  septa  radiate. 

An  independent  thecal  wall,  however,  is  found  to  be  wanting  in  the 
corallum  of  larval  polyps,  the  epitheca  being  in  no  way  comparable  with  this. 
In  the  simple  corallum,  as  far  as  reared,  the  septa  remain  free  and  exposed 
peripherally,  except  in  so  far  as  they  may  be  united  by  synapticular  growths 
or  covered  by  the  epitheca  (see  plates  4  and  5).  Likewise  the  outer  septal 
edges  of  the  marginal  polyps  in  a  colony  extend  for  a  considerable  vertical 
height,  exposed  all  the  way. 

SEPTA. 

The  detailed  characteristics  of  the  septa  have  been  already  described,  so 
that  it  only  remains  to  discuss  their  arrangement  and  relationships. 


ADULT  COLONY.  47 

All  the  septa  of  a  corallite  are  well  developed,  both  as  regards  their 
thickness  and  radial  extent ;  even  the  members  of  the  outermost  cycle  extend 
centrally  for  about  half  the  radius  of  the  calice,  and  are  but  little  narrower 
than  the  others.  The  individual  septa  are  usually  thicker  than  the  width 
of  the  interseptal  loculi  separating  one  from  another,  so  that,  compared  with 
most  other  corals,  the  septal  system  is  very  compact,  and  the  calicinal  cavity 
as  a  whole  is  correspondingly  diminished.  The  septa  vary  but  slightly  in 
thickness  according  to  the  cycle  in  which  they  belong. 

The  outer  septa  are  not  strictly  radial  throughout  their  transverse 
length,  but  incline  along  their  inner  edges  and  fuse  at  intervals  with  the 
lateral  faces  of  the  septa  of  the  second  and  third  cycles,  thus  forming  groups 
of  three,  five,  or  rarely  seven  (fig.  2,  p.  13).  The  grouping,  however,  is  not 
always  obvious  at  the  surface  of  a  colony.  Thus  Verrill  (1901,  p.  154)  states 
that  the  septa  of  the  Bahama  representatives  are  nearly  all  straight  and 

aaAA 

1      X    I         fx    nxi    ixmxnxi    ,1x111x11x111x1;^ 

Fig.  4. — Grouping  of  the  septa  within  a  sextant  (i-l)  according  as  one,  three,  five,  or  seven  septa  are  present. 
The  Roman  numerals  l-iii  indicate  the  order  of  the  septa  and  x  the  exosepta. 

seldom  fused.     It  is,  however,  always  a  marked  characteristic  in  sections  of 
the  corallum  from  a  little  below  the  surface  downward  (plate  10,  fig.  64). 

When  examined  in  detail  the  grouping  is  found  to  be  as  follows :  The 
twelve  largest  septa,  constituting  the  primary  and  secondary  orders,  extend  as 
far  as  the  middle  of  the  calice  and  are  united  directly  with  the  columella. 
Of  these  the  six  primary  septa  pass  uninterruptedly  all  the  way  from  the 
periphery  to  the  columella  without  any  connection  with  the  others  except,  of 
course,  through  union  by  synapticula.  The  six  secondary  septa,  on  the  other 
hand,  are  also  radial,  but  with  them  are  united  the  members  of  the  third 
order,  which  never  reach  the  columella.  The  members  of  the  fourth  or  outer- 
most cycle  fuse  one  on  each  side  with  a  tertiary  septum.  Where  only  one 
secondary  septum  occurs  within  a  primary  system  the  two  outermost  septa  are 
fused  with  it,  and  a  grouping  of  three  is  produced.  Where  a  tertiary  septum 
and  a  secondary  septum  are  within  a  sextant  the  grouping  is  in  fives,  and 
where  two  tertiary  septa  and  a  secondary  occur  the  grouping  is  in  sevens. 
The  relationships  are  clearly  shown  in  the  series  of  figures  above  and  also  in 
fig.  2,  p.  13. 


48  SIDERASTREA    RADIANS. 

In  the  course  of  development  of  the  septa  it  is  shown  that  from  the 
beginning  the  exosepta  always  constitute  the  outermost  cycle,  and  are  fused 
with  the  entosepta  immediately  preceding  them.  Thus,  when  only  two  cycles 
of  septa  are  present  (plate  4,  fig.  23),  the  outer  exosepta  fuse  with  the  inner 
entosepta.  Where  three  cycles  are  developed  (plate  5,  fig.  28)  the  exosepta 
unite  with  second-cycle  entosepta ;  where  fotir  cycles,  they  fuse  with  the  third- 
cycle  entosepta.  At  the  early  stage  of  development,  represented  in  plate  5, 
fig.  28,  the  second-cycle  entosepta  are  fused  with  the  first-cycle  entosepta 
instead  of  with  the  columella  as  in  the  adult.  The  union  here  takes  place 
in  such  a  manner  as  to  give  a  decided  bilateral  symmetry  to  the  corallum. 

The  hexameral  cyclic  plan  of  the  septa  is  not  so  manifest  in  S.  radians 
as  in  many  corals,  partly  owing  to  the  incompletion  of  the  third  and  fourth 
cycles  ;  but  with  a  little  care  it  can  be  established  by  the  differences  in  size 
of  the  septa  and  their  other  relationships.  In  mature  corallites  two  inner 
alternating  orders  (primary  and  secondary)  of  six  septa  each  are  always 
present,  while  the  third  and  fourth  orders  are  represented  by  a  varying  num- 
ber of  septa.  Owing  to  this  varying  incompleteness  of  the  last  two  orders  a 
study  of  the  septal  arrangement  yields  some  interesting  results  in  connection 
with  the  manner  in  which  the  later  additions  of  septa  take  place,  and  also  in 
the  relationships  of  the  entosepta  and  exosepta. 

The  hexameral  plan  is  found  to  be  strictly  followed  so  far  as  concerns 
the  first  and  second  orders,  and  for  the  third  and  fourth  orders  so  far  as  these 
are  developed.  Were  the  third  and  fourth  orders  to  be  completed  the  total 
number  would  be  48,  arranged  in  the  formula  6,  6,  12,  24;  but  this  number 
appears  to  be  rarely,  if  ever,  reached  in  S.  radians^  though  frequently 
exceeded  in  S.  siderea.  In  one  of  the  largest  corallites  44  septa  were  present, 
but  the  numbers  generally  vary  from  30  to  36. 

The  six  members  of  the  primary  cycle  are  readily  separable  from  the 
others  on  account  of  their  greater  radial  extent  and  thickness ;  they  pass  all 
the  way  from  the  thecal  wall  to  the  columella  without  any  of  the  other  septa 
coalescing  with  them.  They  serve  to  distinguish  the  six  primary  systems 
within  which  are  enclosed  varying  numbers  of  smaller  septa.  The  outermost 
exosepta  are  also  recognizable  by  their  smaller  size,  one  occurring  between 
every  two  adjacent  members  of  the  first,  second,  and  third  orders.  Less  cer- 
tainty prevails  in  distinguishing  the  members  of  the  second  and  third  cycles, 
though  frequently  the  second-cycle  septa  are  larger  and  more  prominent  than 
those  of  the  third. 

In  mature  corallites  three  septa  at  least  are  always  present  within  each 
primary  system,  while  there  may  be  five  or  seven  (fig.  2,  p.  13).     Where  only 


ADULT   COLONY.  49 

three  septa  occur  in  a  system  the  middle  member  belongs  to  the  second  order, 
and  the  two  lateral  ones  are  exocoelic;  where  five  septa  are  present,  one 
belongs  to  the  second  order,  another  to  the  third,  and  the  three  alternating 
with  these  to  the  exocoelic  cycle  ;  in  the  rarer  instances  in  which  seven 
septa  are  developed  the  middle  member  is  a  second-order  septum,  the  two 
next  in  size  are  tertiary  septa,  and  the  four  alternating  septa  belong  to  the 
fourth  or  exocoelic  cycle.  The  members  of  the  outermost  cycle  fuse  in  an 
interrupted  manner  by  their  inner  ends  with  the  members  of  the  second 
and  third  cycles,  but  not  with  those  of  the  primary  cycle  ;  the  tertiary  septa 
in  their  turn  unite  with  the  secondary. 

The  actual  ordinal  relationships  of  the  septa  can  be  best  understood 
when  taken  in  connection  with  the  mesenteries,  as  revealed  in  transverse 
sections  of  the  polyp. 

Plate  6,  fig.  34,  represents  a  transverse  section  from  the  lower  stomo- 
daeal  region  of  a  partly  retracted  decalcified  polyp.  The  drawing  was  made 
with  the  help  of  a  camera  lucida,  but  in  the  process  of  decalcification  and  the 
preparation  of  the  section  slight  displacements  of  the  lamellae  have  taken 
place.  Still  the  general  relationships  remain  as  in  the  living  condition,  and 
for  all  practical  purposes  the  invaginations  of  the  polypal  wall  correspond 
with  the  septa  which  have  been  dissolved  away.  Fig.  2,  p.  13,  is  a  diagram- 
matic representation  of  the  septa  of  the  same  polyp,  the  synapticula  and 
spinous  projections  being  omitted  for  the  sake  of  clearness. 

In  the  transverse  section  (plate  6,  fig.  34)  a  large  septal  invagination 
(entosepta,  i,  11,  iii)  occurs  within  the  interspace  inclosed  by  the  two 
members  of  each  pair  of  mesenteries,  and  a  smaller  one  in  the  interspace 
between  every  two  adjacent  pairs  (exosepta,  x).  The  entosepta  inclosed  by 
the  two  pairs  of  directive  mesenteries  are  known  as  directive  septa,  but  in 
the  corallite  itself  there  is  nothing  to  distinguish  these  from  the  other 
primary  entosepta.  They  indicate  the  dorso- ventral  (postero-anterior,  sulculo- 
sulcar)  axis  of  the  corallite,  and  can  only  be  determined  with  certainty  when 
in  association  with  the  mesenteries. 

The  six  interspaces  between  the  six  primary  mesenterial  pairs  contain 
a  variable  number  of  pairs  of  mesenteries  :  the  two  ventral  systems  each 
contain  one  mesenterial  pair  (11),  the  right  middle  interspace  three  pairs 
(hi,  II,  hi),  and  the  remainder  two  pairs  each  (iii,  11).  The  six  pairs  of 
secondary  mesenteries  (11)  are  easily  distinguished  from  the  tertiary  mesen- 
teries (hi)  by  their  greater  size  and  more  central  position.  Of  the  latter 
there  are  only  five  pairs  in  this  particular  polyp  instead  of  twelve,  as 
considerations  of  hexameral  symmetry  would  suggest. 


50  SIDERASTREA    RADIANS. 

The  septa  are  therefore  related  to  the  mesenteries  in  the  following 
manner : 

The  six  primary  septa  are  inclosed  within  the  entocceles  of  the  primary 
cycle  of  six  pairs  of  complete  mesenteries,  the  six  secondary  septa  within  the 
entocceles  of  the  second  cycle  of  mesenteries,  the  five  tertiary  septa  within  the 
entocceles  of  the  third  cycle,  while  the  members  of  the  outermost  or  quater- 
nary cycle  of  septa  are  all  exocoelic,  one  between  each  two  pairs  of  mesenteries. 
The  primary,  secondary,  and  tertiary  septa  are  all  entosepta ;  the  members 
of  the  fourth  cycle  are  exosepta.  Further,  the  number  of  exosepta  equals 
the  sum  of  the  entosepta,  and  the  number  of  cycles  of  septa  is  one  more  than 
the  number  of  cycles  of  mesenteries.  Considering  the  septa  as  entosepta  and 
exosepta,  the  true  septal  formula  is  6,  6,  x,  12  +  x,  where  x  may  be  any 
number  from  one  to  twelve.  The  variations  in  the  number  of  septa  within 
the  different  calices  are  in  pairs — an  entoseptum  and  an  exoseptum — and  affect 
only  the  members  of  the  third  cycle  and  their  corresponding  exosepta.  These 
two  cycles  always  show  a  corresponding  variation  which  is  not  explicable  on 
the  usual  supposition  that  the  cycles  of  septa  are  developed  in  the  order  of 
their  importance.  The  order  of  appearance  of  the  septa  and  the  relationships 
of  the  entosepta  and  exosepta  will  be  discussed  more  fully  in  connection  with 
the  development  of  the  septa  in  larval  polyps. 

It  will  be  shown  that  morphologically  the  exosepta  are  to  be  regarded 
not  so  much  as  a  separate  cycle,  appearing  after  the  others  are  established, 
but  rather  as  the  bifurcated  continuations  of  the  original  cycle  of  exosepta,  or 
perhaps  as  new  formations  arising  along  with  the  entosepta  which  they 
inclose ;  each  entoseptum  has  an  exoseptum  corresponding  with  it,  which  is 
formed  nearly  at  the  same  time.  Bxosepta  are  found  to  arise  along  with 
each  cycle  of  entosepta  or  even  with  each  individual  entoseptum ;  but  as  new 
cycles  of  entosepta  are  formed  they  are,  as  it  were,  shifted  outwardly,  so  as 
always  to  constitute  the  outermost  cycle.  Until  the  adult  condition  is  reached 
the  exosepta  are  but  temporary  predecessors  of  the  permanent  entosepta. 
The  same  relationship  is  found  to  hold  between  the  entotentacles  and  the 
exotentacles. 

The  above  relationship  between  the  cycles  or  orders  of  septa  and  mesen- 
teries holds  for  most  species  of  corals  which  have  been  examined.  In  certain 
forms  the  exosepta  are  absent,  e.  g.^  Pectinia^  Manicina^  when  the  cycles  of 
septa  correspond  in  number  with  the  cycles  of  mesenteries. 

Where  the  hexameral  cyclic  sequence  of  a  corallite  is  not  completed  it 
is  preferable  to  speak  of  the  exosepta  merely  as  exosepta  or  as  septa  of  the 
outermost  cycle,  not  as  a  third  or  a  fourth  cycle;  for  ontogenetically  som? 


ADULT   COLONY.  5 1 

belong  to  the  third  cycle  and  some  to  the  fourth.  They  have  no  ordinal  value 
comparable  with  the  entosepta. 

In  descriptive  works  on  corals,  when  giving  the  number  of  septa 
characteristic  of  any  calice,  it  is  usual  to  consider  all  the  inner  cycles  as 
hexamerously  complete  and  then  to  regard  all  the  missing  septa  as  wanting 
from  the  last  cycle.  Thus  Milne-Bdwards,  in  describing  the  septa  of  the 
present  species,  says  :  ''  Three  cycles  of  septa  complete,  and,  in  general,  a 
variable  number  of  a  fourth  cycle."  Also  Verrill  (1901,  p.  153):  "They 
[the  septa]  form  three  complete  cycles,  with  part  of  the  fourth  cycle  devel- 
oped, so  that  the  number  is  usually  36  to  40."  From  the  relationships  here 
set  forth,  and  more  fully  discussed  in  connection  with  the  development  of 
the  septa,  it  is  clear  that  such  cyclic  plans  do  not  express  the  true  ordinal 
or  morphological  relationships  of  the  septa ;  the  last  and  penultimate  cycles 
vary  in  the  same  degree,  and  the  latter  contains  both  entosepta  and 
exosepta. 

The  septal  invaginations  on  plate  6,  fig.  34,  indicate  how  the  exosepta 
in  most  instances  fuse  with  the  third  septa,  but  in  the  two  ventral  systems, 
where  no  members  of  the  third  cycle  are  developed,  they  unite  instead  with 
the  secondary  septa.  In  the  serial  sections  the  invaginations  also  show  that 
the  fusion  of  the  exosepta  with  the  entosepta  is  not  continuous  throughout 
their  vertical  length,  but  is  effected  only  at  somewhat  regular  intervals. 

The  section  of  a  fragment  of  a  corallum  represented  on  plate  10,  fig.  64, 
includes  the  whole  of  one  corallite  and  portions  of  the  six  surrounding 
corallites,  and  shows  their  relationships  to  one  another.  The  thecal  wall 
separating  one  corallite  from  those  adjacent  to  it  is  seen  to  be  very  limited 
in  thickness.  At  this  level  in  the  complete  calice  twelve  septa,  representing 
the  first  and  second  cycles,  are  united  directly  with  the  columella.  The  six 
primary  septa  are  distinguished  by  the  fact  that  they  extend  all  the  way 
from  the  periphery  to  the  columella  without  fusion  with  any  of  the  smaller 
septa.  In  three  of  the  primary  systems  there  are  only  three  septa — a  second- 
cycle  entoseptum  and  two  complete  exosepta  fused  with  it ;  in  the  remaining 
three  sextants  are  five  septa — a  second-cycle  entoseptum  with  one  exoseptum 
and  a  third-cycle  entoseptum  fused  with  it,  the  latter  having  two  exosepta 
in  union  with  it. 

Comparison  may  be  here  made  with  a  somewhat  similar  horizontal  sec- 
tion of  a  corallite  of  S.  radians,  introduced  on  plate  xv  (fig.  12)  of  Agassiz's 
"  Florida  Reefs."  Nine  distinct,  complete  septa  are  represented,  and  in  the 
interspace  between  each  two  occurs  a  group  of  three  septa,  the  two  lateral 
united  with  the  middle,  which,  in  its  turn,  extends  to  the  columella.     The 


52  SIDERASTREA    RADIANS. 

arrangement  of  the  septa  in  groups  of  three  is  continued  all  the  way  round, 
and  the  hexameral  plan  is  altogether  obscured.  Such  an  interpretation  of 
the  septal  plan  is  at  variance  with  what  is  established  above  for  the  Jamaica 
representatives  of  the  same  species. 

As  seen  at  the  surface  of  a  colony  the  septal  edges  extend  almost  hori- 
zontally for  a  short  distance  from  the  periphery,  and  are  then  inclined  down- 
ward and  inward  somewhat  sharply  to  meet  the  columella  which  forms  the 
floor  of  the  middle  of  the  calice.  The  actual  depth  of  the  calice  varies  some- 
what in  different  colonies,  but  is  usually  about  2  mm. 

SYNAPTICULA. 

Viewed  with  a  lens  from  above,  the  septa  are  seen  to  be  joined  to  one 
another  laterally  by  thick  transverse  bars — the  synapticula.  In  any  trans- 
verse section  of  a  calice  one  to  three,  rarely  four,  synapticula  are  seen  crossing 
the  space  between  every  two  adjacent  septa,  but  are  limited  in  their  distribu- 
tion to  the  peripheral  half  of  the  calice  (plate  lo,  fig.  64).  In  a  view  of  the 
lateral  surface  of  the  larger  septa  they  are  found  to  be  arranged  in  two  or 
three,  rarely  four,  somewhat  irregular  vertical  rows,  each  synapticulum  being 
circular  or  oval  in  section,  and  projecting  at  right  angles  from  the  septal 
surface  (plate  10,  fig.  63).  The  presence  of  so  many  connections  between  the 
septa  gives  a  porous  or  reticular  character  to  the  more  peripheral  part  of  the 
calice. 

The  synapticula  are  formed  by  the  ultimate  fusion  across  an  interseptal 
loculus  of  two  opposite  granulations  on  the  faces  of  adjacent  septa,  the  calca- 
reous matter  being  deposited  in  a  manner  similar  to  that  of  other  parts  of  the 
corallite.  In  Siderastrea  it  is  only  the  granules  in  restricted  spots  on  the 
septal  faces  which  become  thus  enlarged.  The  synapticula  have  a  limited 
distribution,  and  the  other  granulations  on  the  septal  face  remain  compara- 
tively small,  there  being  no  intermediate  examples.  It  is  clear  that  in  the 
course  of  their  growth  the  enlarging  granules  must  first  indent  the  skeleto- 
trophic  tissues  which  line  them,  then  bring  the  two  opposite  layers  together, 
and  finally  perforate  them,  as  shown  on  plate  6,  fig.  34  ;  also,  as  described  on 
p.  28,  the  mesentery  inclosed  within  the  loculus  is  perforated  at  the  same  time. 

Sections  through  the  synapticula  show  that  they  are  formed  as  lateral 
processes  of  the  septa,  either  by  an  extension  of  the  septal  trabeculae  or  from 
a  center  of  calcification  of  their  own.  In  the  former  case  a  boundary  in  the 
middle  of  the  synapticulum  indicates  where  the  bundles  of  fibro-crystals  from 
one  septum  have  met  those  from  the  opposite  septum.     In  the  other  case  it 


ADULT  COLONY.  53 

appears  as  if  the  fibers  from  tlie  synapticulum  itself  were  bracing  tbe  two 
septa.  The  first  are  known  as  false  synapticula  and  the  second  as  true 
synapticula,  but,  as  all  recent  writers  have  pointed  out,  there  is  no  morpho- 
logical distinction  between  the  two.  Both  kinds  appear  in  any  section  of  a 
corallite.  Whether  one  or  the  other  form  is  present  depends  mainly  upon  the 
interval  between  the  two  septa.  Where  sufficiently  close,  as  towards  the 
middle  of  the  calice,  the  interseptal  space  can  be  bridged  without  the  forma- 
tion of  a  new  center  of  calcification,  while  such  is  necessary  when  the  interval 
is  wide,  as  towards  the  periphery. 

Miss  Ogilvie  (1897,  p.  168)  considers  that  in  J^ung-ta  the  synapticula  are 
formed  by  special  interseptal  invaginations  of  the  basal  wall  of  the  polyp 
subsequent  to  the  septal  upgrowths,  without  ever  perforating  the  mesenteries, 
and  assumes  such  to  be  the  case  with  Siderastrea.  Bourne  (1886,  p.  47)  has 
already  shown  that  in  Fungia  the  mesenteries  are  really  pierced  by  the 
synapticula,  Fowler  (1888,  p.  8)  has  accomplished  the  same  for  Stephano- 
phyllia  formosissinia^  and  I  for  Siderastrea  siderea  (1902,  p.  487).  The  con- 
ditions revealed  by  the  liberated  lamella  on  plate  6,  fig.  33,  prove  that  the 
synapticula  are  the  products  of  definite  areas  of  the  original  skeletogenic  layer 
producing  the  septa,  not  of  special  secondary  basal  upgrowths.  Delage  & 
Herouard  (1901)  have  accepted  Miss  Ogilvie's  interpretation  of  synapticular 
formation,  giving  diagrammatic  figures  to  illustrate  how  the  upgrowth  is 
supposed  to  take  place. 

COLUMELLA. 

The  columella  is  represented  in  the  mature  calice  by  a  more  or  less 
compact,  column-like  structure  forming  the  floor  of  the  middle  of  the  calice. 
In  decalcified  polyps  it  is  found  to  have  elevated  the  central  part  of  the 
skeletogenic  tissues  in  a  tubular  manner  considerably  beyond  the  peripheral 
parts.  Its  superficial  appearance  varies  much  in  different  calices  ;  it  may 
be  either  papillose  or  smooth,  dependent  upon  the  degree  of  calcification  of 
the  colony.  Sometimes  only  one  large,  tooth-like  papilla  occurs,  or  there 
may  be  two  close  together ;  in  others,  again,  there  are  two  or  three  large  or 
chief  tubercles  along  with  several  minute  projections,  or  it  may  be  formed 
wholly  of  small,  scarcely  perceptible  granules,  when,  to  the  naked  eye,  it 
appears  compact  and  smooth.  This  latter  condition  is  found  in  coralla  of 
which  all  the  parts  are  strongly  calcified.  In  these  the  columella  is  conspic- 
uous, raised  for  a  short  distance  above  the  septa,  and  its  free  surface  nearly 
smooth. 

Where  several  prominent  tubercles  are  present  some  seem  as  if  con- 
tinuous with  the  teeth  of  the  septa,  suggesting  that  the  columella  is  formed 


54  SIDERASTREA    RADIANS. 

from  the  united  edges  of  the  primary  and  secondary  septa.  Others  of  the 
projections  seem,  however,  to  be  quite  independent  of  the  septa.  Secondary 
calcareous  matter  is  deposited  among  the  projections,  and  this  gives  the  more 
compact  character  to  the  structure  as  a  whole.  The  section  represented  on 
plate  lo,  fig.  65,  shows  the  relationship  clearly.  The  columella  is  here 
quite  solid  and  distinct,  and  is  structurally  the  same  as  the  septa  united  with 
it,  having  its  own  centers  of  calcification.  In  sections  a  little  higher  it  is 
spongiform,  as  the  trabeculae  are  free  from  one  another. 

With  such  structural  details  alone  available  it  is  practically  impossible 
to  say  whether  the  columella  of  Siderastrea  is  a  true  or  a  false  columella 
(pseudocolumella)  as  these  terms  are  understood  in  coral  literature.  Miss 
Ogilvie  (1897,  p.  179)  describes  it  as  a  ^'paliform  pseudocolumella."  A  true 
columella  is  considered  to  arise  as  an  independent  structure  from  the  middle 
of  the  basal  plate,  though  the  septal  edges  may  secondarily  unite  with  it.  A 
false  or  pseudocolumella,  on  the  other  hand,  is  an  irregular  skeletal  tissue 
formed  from  union  of  the  inner  or  central  ends  of  the  septa,  sometimes  bound 
together  by  a  secondary  deposit.  Recourse  to  the  developing  coralla  shows 
the  true  nature  of  the  columella  in  S.  radians.  As  described  later,  and 
illustrated  by  the  figures  on  plates  4  and  5,  independent  skeletal  upgrowths 
are  first  formed  from  the  basal  plate,  but  later  come  into  intimate  relation- 
ship with  similar  formations  at  the  edges  of  the  septa,  and  afterwards  the 
two  groups  are  united  into  a  solid,  compact  column  by  a  deposit  of  calcareous 
matter. 

Developmentally,  therefore,  the  columella  of  Siderastrea  is  a  true  colu- 
mella, compact  all  the  way,  or  compact  below  and  spongy  above. 

Histologically  the  columella  usually  shows  two  or  three  large  circular 
trabeculse  arranged  in  a  row  and  surrounded  by  a  number  of  smaller  tra- 
beculae (plate  10,  fig.  65).  The  larger  probably  represent  the  true  elements 
of  the  columella,  which  arise  directly  from  the  basal  plate,  while  the  smaller 
are  the  septal  teeth  which  also  take  part  in  its  constitution. 

Viewed  from  the  surface  the  columella  is  usually  nearly  circular  in 
outline  and  affords  no  aid  in  determining  the  principal  axis  of  the  corallite ; 
but  in  sections  some  distance  below  the  surface  it  generally  presents  a  longer 
and  a  shorter  axis,  and  the  former  evidently  corresponds  with  the  directive 
or  principal  axis  of  the  corallite.  To  each  extremity  of  the  longer  axis  a 
septum  of  the  first  order  is  attached,  and  the  two  are,  no  doubt,  the  directive 
septa,  though,  owing  to  the  difference  in  the  number  of  septa  on  each  side, 
they  are  not  always  in  the  same  plane  (plate  10,  fig.  64).  The  oval  columella 
may  thus  afford  an  important  aid  in  the  orientation  of  the  septa  and  of  the 
corallites  in  a  colony. 


ADULT   COLONY.  55 


DISSEPIMENTS. 


The  dissepiments  are  extremely  thin  and  delicate  transverse  partitions 
which  serve  to  cut  ojff  the  living  polyps  from  the  dead  part  of  the  skeleton 
below.  In  section  they  are  only  0.0 1  mm.  thick.  Along  with  the  columella 
they  serve  for  the  time  being  as  a  support  for  the  basal  part  of  the  polyp,  the 
dissepiment  being  morphologically  comparable  with  the  basal  plate  of  the 
larval  polyp.  In  some  cases  the  partitions  are  horizontal,  but  in  others  they 
are  deeply  convex  upwardly.  They  are  situated  at  somewhat  different  levels 
in  different  interseptal  loculi,  and  in  the  same  loculus  vary  from  0.25  mm.  to 
0.5  mm.  apart.  The  distance  apart  of  two  adjacent  dissepiments  in  a  loculus 
represents  a  period  of  growth  of  the  polyp  in  its  march  upward,  the  polyps 
throughout  their  lifetime  retaining  approximately  the  same  vertical  length 
and  transverse  diameter.  As  a  result  of  the  very  narrow  interseptal  loculi  in 
Siderastrea  the  dissepiments  are  rather  insignificant  features  of  the  corallum, 
but  as  a  basal  support  they  are  probably  as  important  to  the  polyp  as  in  other 
species  of  corals  in  which  they  are  more  prominent.  Occasionally  a  dissepi- 
ment is  found  intersected  by  a  synapticulum,  when  both  may  be  considered  as 
constituting  the  basal  support  of  the  particular  lamella.  Such  was  probably 
the  case  in  the  interseptal  lamella  represented  in  plate  6,  fig.  33.  Usually- 
the  lamellae  present  a  more  nearly  straight  basal  extremity  than  is  here  repre- 
sented, showing  that  the  polyp  is  cut  off  below  in  a  regular  manner  by  the 
formation  of  dissepiments  which  alternate  with  the  synapticula.  The  dissepi- 
ments may  thus  correspond  or  alternate  with  the  synapticula,  the  latter  being 
formed  independently  and  in  advance  of  them. 

The  dissepiments  are  easily  studied  in  tangential  sections  of  corallites, 
and  it  is  found  that  their  histological  structure  is  different  from  that  of 
septa.  They  possess  no  centers  of  calcification,  but  are  thin  and  laminated, 
composed  of  fibro-crystals  standing  at  right  angles  to  the  surface.  They  thus 
recall  the  microscopic  structure  of  the  basal  plate  and  epitheca,  and  like  these 
they  are  lined  by  the  calicoblasts  of  the  polyp  only  on  one  side. 

Miss  Ogilvie  (1897,  p.  178)  assumes  that  the  synapticula  and  not 
the  dissepiments  constitute  the  principal  basal  support  of  the  polyp  in 
Siderastrea.  This  would  follow  were  the  synapticula  formed,  as  she  assuines, 
within  special  interseptal  invaginations  of  the  aboral  body  wall ;  but,  as 
shown  on  p.  53,  such  is  not  their  origin.  They  are  produced  by  special  areas 
of  the  septal  invagination,  ultimately  resulting  in  perforation  of  the  polypal 
wall.  As  the  synapticula  actually  perforate  the  polyp  they  may  be  conceived 
as  also  affording  it  support ;  but  only  incidentally,  as  happens  to  be  the  case 


56  SIDERASTREA    RADIANS. 

in  the  lamella  shown  in  plate  6,  fig.  33,  is  this  basal.     The  synapticula  do 
not  take  the  place  nor  assume  the  function  of  dissepiments. 

EPITHECA  AND  BASAL  PLATE. 

The  most  careful  observation  of  the  margin  of  adult  coralla  fails  to 
reveal  the  presence  of  an  epitheca  or  covering  of  the  peripheral  septal  edges. 
This  is  somewhat  remarkable  considering  that  such  a  skeletal  formation  is 
developed  in  young  polyps  reared  from  the  larva,  and  this  whether  the 
polyps  are  isolated  or  growing  contiguous  to  others  (plates  4,  5).  In 
these  larval  polyps,  however,  the  extent  of  the  upward  growth  of  the  epitheca 
has  been  found  to  vary  greatly.  In  some  specimens,  apparently  stationary 
as  regards  growth,  it  formed  a  comparatively  high  external  wall  to  the  polyp, 
wrinkled  and  diminishing  somewhat  in  diameter  from  the  base  upwards,  but 
overtopping  all  the  septa  (plate  5,  fig.  27),  while  in  others,  the  largest, 
actively  growing  corals,  it  was  either  absent  or  represented  only  by  a  low, 
narrow  rim  (plate  5,  fig.  28).  Evidently  it  is  a  structure  of  importance 
only  in  the  early  stages  of  growth. 

Where,  as  frequently  happens,  a  part  of  the  growing  margin  of  a  colony 
is  free  from  the  incrusted  object,  a  flat,  plate-like  deposit  covers  and  unites 
externally  the  basal  edges  of  the  septa.  Such  a  formation  by  the  bud-polyps 
will  correspond  with  the  basal  plate  of  the  larval  polyp,  but  its  peripheral 
edge  is  rarely,  if  ever,  upturned  so  as  to  cover  the  free  vertical  edges  of  the 
septa,  as  in  the  case  of  an  epitheca  proper. 


POSTLARVAL  DEVELOPMENT. 
LARVA. 

Five  diflferent  colonies  of  S.  radians  were  collected  in  Kingston  Harbor, 
Jamaica,  on  the  6tli  of  July,  many  of  the  polyps  of  which  contained  free 
planulae.  Although  similar  colonies  had  been  obtained  from  this  locality 
on  former  occasions,  and  examined  with  regard  to  their  fertility,  this  was  the 
first  time  that  larvae  were  secured.  On  a  second  visit  a  week  later  two  or 
three  other  ripe  colonies  were  collected,  and  others  which  on  being  sectionized 
were  found  to  contain  ova. 

On  every  colony  the  fertile  polyps  were  in  somewhat  restricted  patches, 
not  all  the  individuals  coming  to  maturity  at  the  same  time.  One,  two,  or 
three  planulae  were  plainly  visible  through  the  nearly  transparent  tissues  of 
the  parent,  and  were  often  observed  to  pass  into  the  tentacles,  which  thereby 
became  greatly  distended,  remaining  so  even  when  the  polyps  were  retracted. 
At  times  the  planulae  would  glide  into  the  lower  regions  of  the  polypal  cavity, 
and  were  then  lost  to  view.  While  within  the  parent  cavity  it  was  impossible 
to  determine  whether  the  movement  of  the  larvae  was,  like  that  of  the  food 
particles,  dependent  upon  the  ciliary  activity  of  the  lining  endoderm  or  was 
a  result  of  the  larva's  own  activity.  Immediately  upon  being  set  free,  how- 
ever, the  larvae  were  able  to  swim  about,  the  ectoderm  being  already  provided 
with  a  layer  of  cilia. 

The  larvae  were  shot  out  suddenly,  but  the  actual  place  of  extrusion  was 
not  determined,  although  prolonged  observation  was  made.  From  their  per- 
sistent entrance  into  the  tentacles  it  would  seem  that  they  made  their  escape 
through  these  organs,  and  not  through  the  mouth,  which  was  usually  closed 
and  depressed.  In  other  instances,.^.  ^.,  Manicina  areolata  and  Favia 
fragum^  the  sexual  products  and  larvae  have  been  seen  to  be  given  out 
through  the  oral  aperture,  the  proceeding  being  accompanied  by  a  peculiar 
jerking  motion  of  the  adult  polyp.  Von  Koch  (1897),  however,  found  the 
larvae  of  Caryophyllia  cyathus  to  be  expelled  through  the  tips  of  the  tentacles, 
and  Lacaze-Duthiers  (1873,  p.  308)  occasionally  observed  the  same  in  Astroides 
calycularis.  Sections  show  that  the  knobbed  tip  of  the  tentacles  in  Sideras- 
trea  is  without  any  permanent  terminal  aperture. 

Larvae  were  freely  extruded  at  the  time  of  collection  of  the  corals,  and 
continued  to  be  discharged  from  time  to  time  for  about  a  month.  Under  ordi- 
nary conditions  one  or  two  would  be  set  free  at  intervals,  but  upon  disturbance 

57 


58  SIDERASTREA    RADIANS. 

of  the  colony  a  score  or  so  would  be  sliot  out  together.  Often  when  the  disc 
had  become  retracted  a  larva  would  remain  within  the  upper  peripheral  region 
of  the  polyp,  fixed  between  the  edges  of  a  septum  and  the  wall  of  a  tentacle. 

The  larvae  were  able  to  swim  about  immediately  upon  being  discharged, 
and  gyrated  through  the  water  first  in  one  direction  and  then  in  another. 
There  was  evidence  of  a  feeble  negative  geotropism.  During  the  first  day 
they  kept  near  the  surface  of  the  water,  or  gathered  around  the  sides  of  the 
vessels  ;  afterwards  they  traversed  the  water  as  a  whole,  though  in  the  main 
keeping  near  the  surface. 

To  the  unaided  eye  some  of  the  planulse  appeared  as  minute,  opaque- 
white,  spheroidal  bodies  ;  others  were  oval ;  but  the  majority  were  elongated 
and  pear-shaped,  as  shown  in  plate  i ,  fig.  i .  Under  the  microscope  the  ciliation 
was  found  to  be  uniform  over  the  whole  surface.  The  broader  pole  (oral) 
was  deeply  pigmented  and  invariably  posterior  in  swimming ;  no  oral  aperture 
could  be  detected  at  first,  but  extrusions  of  Zooxanthellse,  yolk,  and  cell 
debris  took  place  later,  showing  that  the  mouth  was  already  functional. 
Under  the  microscope  the  brownish  yellow  of  the  broad  end  was  seen  to  be 
due  to  the  presence  of  numerous  Zooxanthellas  within  the  ectodermal  la^^er ; 
otherwise  the  ectoderm  was  colorless,  while  the  endoderm  was  dark  and  non- 
transparent.  The  narrow  anterior  end  (aboral)  was  perfectly  colorless,  its 
ectoderm  being  free  from  commensal  algae.  In  some  larvae  the  colorless  end 
(aboral)  was  larger  than  the  other.  The  elongated  specimens  were  able  to 
retract  and  extend  themselves,  and  upon  irritation  or  preservation  altered 
their  shape  so  as  to  become  nearly  spherical.  The  ordinary  pear-shaped 
individuals  were  about  2  mm.  long. 

Throughout  their  free  existence  the  majority  of  the  larvae  remained  as 
wholly  opaque  objects,  and  no  trace  of  mesenterial  divisions  could  be  seen 
from  the  outside.  Some  specimens,  however,  were  so  far  developed  as  to  be 
distended  at  birth,  or  became  so  shortly  afterwards.  The  tissues  of  these 
latter  were  more  transparent  than  the  rest,  and  eight  mesenterial  divisions 
were  obvious,  though  not  all  of  the  same  vertical  length  (plate  i,  figs.  3  and  4). 
The  latest  extruded  larvae,  after  the  colonies  had  been  kept  under  somewhat 
unfavorable  conditions  for  two  or  three  weeks,  were  nearly  devoid  of  external 
Zooxanthellas  and  therefore  colorless. 

After  the  first  day  or  two  many  of  the  larvae  showed  positive  geotropism 
and  sank  to  the  bottom  of  the  vessels,  lying  there  motionless  ;  later,  some  of 
these  resumed  their  activities.  By  the  evening  of  the  second  day  a  few  had 
fixed  themselves  by  the  narrow,  anterior,  colorless  end  to  the  bottom  or  side 
of  the  vessel,  or  to  objects  placed  within  it.     At  first  the  larvae  would  adhere 


POSTLARVAL  DEVELOPMENT.  59 

by  means  of  the  actual  tip  of  the  narrow  end  (plate  i,  fig.  5) ;  this  would  then 
flatten  out,  and  the  whole  body  become  much  shorter,  a  small  round  or  oval 
aperture  being  visible  at  the  free  extremity  (plate  i,  fig.  6). 

Whether  or  not  any  individual  larv^a  would  settle  seemed  very  uncertain, 
for  out  of  several  hundreds  set  free  comparatively  few  became  permanently 
fixed.  Those  which  settled  appeared  to  be  the  larger,  better  developed 
specimens.  Smaller  vessels,  glass  slides,  and  cover  glasses  floated  by 
means  of  cork  were  placed  within  the  receptacles  so  as  to  afford  additional 
surfaces  for  adhesion  ;  in  other  cases  larvas  were  distributed  wdthin  vessels 
coated  with  paraffin.  If  fixation  were  not  accomplished  within  the  first  few 
days,  it  seemed  to  be  impossible  afterwards,  though  the  larvae  might  continue 
active  for  several  weeks.  In  one  instance  about  a  score  of  planulse  were 
isolated  and  kept  in  a  glass  dish,  and  although  they  appeared  healthy  and 
swam  freely  for  a  period  of  twenty  days,  externally  they  underwent  no  change 
whatever. 

Sometimes  a  larva  would  fix  itself  in  a  position  apart  from  others  ;  but  in 
general  many  would  settle  close  together  at  one  and  the  same  time,  their  walls 
in  some  cases  actually  pressing  one  against  another.  Plate  i,  fig.  5,  represents 
three  larvas  in  the  first  stage  of  fixation,  in  this  case  to  a  small  pebble.  The 
narrow  extremities  nearly  touch,  and  it  is  obvious  that  when  the  larvas  shorten 
and  their  bases  flatten,  the  latter  will  exert  a  mutual  pressure  as  a  result 
of  their  closeness.  In  plate  i,  fig.  6,  seven  larvae  already  settled  and  fully 
expanded  are  represented,  as  seen  from  above  by  reflected  light.  All  were 
closely  adherent  to  a  fragment  of  stone,  and  formed  a  miniature  colony.  To 
the  under  surface  of  a  small  pebble  thirty-eight  other  specimens  were  aggre- 
gated in  groups  of  two,  three,  or  more,  the  members  of  any  group  touching 
along  their  margin.  One  of  these  groups  contained  a  dozen  or  more  young 
polyps,  all  in  contact  with  one  another,  the  mutual  pressure  producing  a 
distortion  of  the  normally  circular  base. 

During  a  single  night  another  group  of  thirty-two  became  adherent  to  the 
surface  of  a  small  glass  dish.  In  this  case  nearly  all  the  members  were  touch- 
ing to  a  greater  or  less  degree.  One  of  the  aggregations,  consisting  of  fifteen 
polyps,  is  represented  in  fig.  5,  p.  60,  as  seen  from  the  under  surface  of  a 
fragment  of  glass  to  which  the  individuals  were  attached.  The  colony  when 
drawn  was  two  or  three  months  old,  and  the  skeleton  was  already  in  process 
of  development,  being  represented  by  a  variable  number  of  septa  and  by  basal 
and  epithecal  formations. 

The  aggregated  larvae,  or  young  polyps,  as  they  may  be  called  after 
settling,  remained  closely  associated  during  their  subsequent  growth,  and  in 


6o  SIDERASTREA    RADIANS. 

all  respects  resembled  a  distinct  colony.  Siderastrea  radians  thus  exhibits 
the  phenomenon  of  a  coral  reaching  its  colonial  stage  by  direct  union  or 
aggregation  of  a  number  of  independent  larvae,  the  larvae  being  at  first  free 
swimming  and  wholly  unconnected  with  one  another.  The  close  association 
is  probably  to  be  understood  as  a  thigmotactic  phenomenon,  and  calls  for  a 
more  extended  study  with  abundant  material.  The  subject  of  "  Aggregated 
Colonies  in  Madreporarian  Corals  "  has  been  more  fully  discussed  in  "  The 
American  Naturalist,"  June,  1902.  It  is  there  shown  that  similar  associa- 
tions are  at  times  met  with  in  other  corals. 

Bxtemally  the  opaque  larvae  seemed  all  alike,  but  sections  reveal  slightly 
different  stages  of  growth  as  regards  the  number  of  mesenteries  and  their 
connection  with  the  stomodaeum.     The  sections  further  demonstrate  that  the 


Fig.  5. — Aggregation  of  fifteen  polyps  derived  from  the  same  number  of  originally  free-swimming  larvae. 
Unly  the  basal  skeletal  appearance  is  represented. 

opacity  is  due  to  the  thickness  of  the  endoderm  and  the  numerous  Zooxan- 
thellae  within  its  cells.  Furthermore,  the  gastro-coelomic  cavity  is  yet 
unformed,  or  very  limited  in  extent,  the  larvae  being  a  nearly  solid  mass  of 
cells.  The  distended  larvae,  on  the  other  hand,  already  showed  from  the 
outside  different  stages  in  the  development  of  the  mesenteries,  and  the 
internal  cavity  was  more  fully  established  {cf.  plate  i,  figs.  3,  4). 

After  fixation  all  the  larvae  became  distended,  the  walls  more  trans- 
parent, and  mesenterial  divisions,  twelve  in  number,  were  rendered  conspicuous. 
From  the  beginning  of  fixed  life  some  few  were  more  advanced  than  others. 
These  were  the  individuals  bom  in  an  already  transparent  state.  On  set- 
tling they  were  nearly  as  large  again  as  the  other  larvae,  and  their  develop- 
ment throughout  was  more  rapid. 

A  few  abnormal  larvae  were  extruded  and  swam  about  like  the  rest. 
The  oral  extremity  of  these  was  double  and  the  aboral  single,  the  bifurcation 
taking  place  about  the  middle  of  the  length  (plate  i,  fig.  2).  Lacaze-Duthiers 
(1873,  p.  312)  likewise  found  such  double  monsters  to  be  somewhat  frequent 


POSTLARVAL  DEVELOPMENT.  6l 

in  Astroides.  In  these  the  oral  end  was  sometimes  bifurcated  and  sometimes 
the  aboral. 

No  trace  of  any  calcareous  deposit  or  skeletal  formation  was  apparent 
in  any  of  the  larvae  before  fixation,  but  calcareous  matter  was  laid  down  soon 
afterwards,  the  distinction  between  basal  disc  and  free  polypal  wall,  or  skeleto- 
genic  and  non-skeletogenic  tissues,  being  then  established  for  the  first  time. 

It  is  convenient  to  describe  the  general  course  of  the  development  and 
character  of  the  young  polyps  before  proceeding  with  the  detailed  growth  of 
the  individual  organs — tentacles,  mesenteries,  and  septa. 

YOUNG  POLYP. 

Some  of  the  larvae  became  adherent  to  small  pebbles,  others  to  pieces  of 
glass,  while  most  were  fixed  to  the  sides  and  bottom  of  the  glass  vessels  in 
which  the  colonies  were  placed.  In  this  last  case  the  vessels  were  sacrificed 
in  order  to  obtain  the  young  polyps  in  a  condition  favorable  for  examination 
under  the  microscope.  By  carefully  breaking  the  glass  suitable  fragments 
were  secured  with  the  polyps  attached,  and  these  could  be  transferred  from 
one  aquarium  to  another,  or  placed  in  dishes  small  enough  to  rest  on  the 
stage  of  the  microscope.  In  this  way  the  growing  polyps  could  be  examined 
at  any  time  from  both  their  upper  and  under  aspects,  their  partial  trans- 
parency allowing  the  development  of  the  mesenteries  and  septa  to  be  followed 
day  by  day.  Thus  the  growth  of  the  polyp  and  the  skeleton  could  be  observed 
together  and  their  various  relationships  studied. 

Once  the  larvae  were  settled  they  seemed  vigorous  and  hardy,  and  scarcely 
any  of  the  young  polyps  arising  therefrom  succumbed,  although  subjected 
to  the  somewhat  adverse  conditions  of  small  aquaria. 

The  different  organs,  polypal  and  skeletal,  began  to  appear  shortly 
after  fixation,  the  various  stages  in  the  development  of  the  tentacles,  mesen- 
teries, and  septa  being  represented  by  the  drawings  and  photographs  on  plates 
1-5.  The  young  polyps  were  early  capable  of  expansion  and  retraction. 
For  the  most  part  they  remained  in  the  expanded  condition,  with  the  tentacles 
stretching  out  horizontally,  or  even  overhanging.  Sometimes  the  column 
would  extend  vertically  upwards  and  remain  equal  in  diameter  throughout ; 
at  other  times  the  lower  part  of  the  walls  would  shrink  over  the  corallum 
and  the  remainder  appear  as  a  narrow  column  upon  a  broad  pedestal.  Upon 
retraction  the  tentacles  still  remained  exposed,  but  restricted  to  the  central 
region  (plate  2).  Even  in  the  adult  the  tentacles  have  been  found  to  remain 
visible  upon  the  fullest  retraction,  the  column  being  incapable  of  folding 
over  them.      On   one   occasion   only  a   larval  polyp  was  found  with  the 


62  SIDERASTREA    RADIANS. 

column  wall  completely  overfolding  the  tentacles  and  covering  nearly  all 
the  disc  (plate  3,  fig.  18). 

After  the  first  few  days  the  polyps  were  able  to  feed  upon  small  pieces 
of  molluscs  or  chastopods  placed  upon  the  disc  by  means  of  forceps  or  a 
needle.  The  tentacles  would  close  upon  the  fragment  and  hold  it  as  the 
forceps  were  withdrawn.  If  the  pieces  offered  were  small  enough  they  were 
taken  bodily  within  the  gastric  cavity,  but  when  too  large  to  be  engulfed  a 
portion  only  would  be  drawn  within  the  stomodseum,  remain  there  for  two  or 
three  hours,  and  then  be  pushed  away  over  the  side  of  the  disc. 

For  some  time  Zooxanthellae  were  present  within  the  ectoderm  of  the 
disc  and  upper  part  of  the  column,  and  rendered  these  regions  nearly  opaque ; 
but  later  they  wholly  disappeared  from  the  outer  layer,  and,  as  in  the  adult, 
were  restricted  to  the  endoderm.  As  the  polyps  grew  larger  their  walls 
became  thinner  and  more  transparent.  The  internal  yellow  cells  were 
thickly  distributed  along  the  two  sides  of  the  mesenteries  and  within  the 
endoderm  of  the  stalk  of  the  tentacles,  but  were  sparse  towards  the  lower 
part  of  the  column.  They  formed  an  important  aid  in  determining  the  course 
of  the  mesenteries  within  the  living  polyps. 

The  young  polyps  displayed  great  power  of  recovery  from  injury.  In 
two  or  three  instances  the  glass  to  which  they  were  attached  was  broken  in 
such  a  way  that  a  polyp  was  completely  divided  into  two,  the  halves  adherent 
to  different  fragments ;  yet  the  two  parts  continued  to  live,  and  each  became 
a  distinct,  though  smaller,  polyp,  nearly  circular  on  healing,  and  restoring 
in  every  way  the  normal  form. 

On  the  polyps  attaining  partial  transparency,  the  internal  cavity  could 
be  seen  to  contain  granular  matter  and  Zooxanthellae  in  a  regular  rapid 
movement,  and,  by  focussing  at  different  heights,  it  was  possible  to  make  out 
the  complete  course  of  the  circulation.  The  particles  passed  directly  upward 
along  the  column,  then  transversely  across  the  disc,  downward  along  the 
endodermal  surface  of  the  stomodaeum,  and  thus  right  into  the  central  cavity 
of  the  polyp ;  they  then  continued  radially  across  the  base  outwards  towards 
its  periphery,  and  once  more  up  the  column.  The  regularity  of  the  course 
was  a  little  disturbed  at  the  opening  of  the  tentacles  into  the  gastric  cavity. 
Some  of  the  particles  would  circulate  around  this  region  in  an  eddy-like 
manner,  or  pass  a  little  within  the  tentacular  cavity,  and  afterwards  be 
wafted  along  in  the  general  current.     The  course  is  indicated  in  fig.  6. 

Occasionally  the  lips  of  the  stomodaeum  would  meet  in  the  middle  and 
remain  open  at  each  end,  but  no  incurrent  or  excurrent  streams  were  ever 
detected   through  the  apertures  thus  formed.     A  bolus  of   organic  debris, 


POSTLARVAL  DEVELOPMENT.  63 

containing  Zooxanthellae,  would  at  times  be  seen  whirling  round  and  round 
within  the  stomodseum,  and  would  afterwards  be  jerked  out  and  lost  to  the 
polyp.     Even  after  three  months  such  extrusions  would  still  take  place. 

The  young  polyps  varied  much  in  their  rate  of  growth,  especially  in 
the  later  stages  ;  indeed,  in  some  instances,  development  seemed  to  be  wholly 
arrested.  The  isolated  polyps  progressed  most  rapidly,  but  among  these 
many  differences  were  recognizable.  The  polyps  forming  colonies  made 
scarcely  any  progress  after  the  first  week  or  two.  In  the  group  represented 
in  plate  i,  fig.  6,  no  increase  beyond  the  six  tentacles  and  twelve  simple 
septa  occurred  up  to  six  weeks,  when  the  colony  was  preserved. 

The  development  of  the  various  organs  was  characterized  by  certain 
well-marked  intervals  of  rest  and  growth.  A  week  or  two  might  pass  without 
any  conspicuous  change  taking  place,  and  then 
progress  would  be  somewhat  rapid.  The  order  of 
appearance  of  the  organs  varied  somewhat  in  dif- 
ferent individuals  ;  at  first  some  organs  of  a  system 
were  developed  a  cycle  at  a  time,  and  the  remainder 
in  successive  pairs  from  one  border  of  the  polyp  to 
the  other.  Perhaps  under  the  artificial  conditions 
the  duration  of  some  of  the  intervals  was  partly 
determined  by  the  state  of  the  water  and  the  amount 

.  ,~    .-^  ,  Fig.  6. — Diagram  showing  the  course  of  the 

and  character  of   the  food   supplied.     Still,   there    circulation  within  the  imemai  cavity  of  a 

were  certain  definite  resting  stages  appearing  in  all     ""*  ^°  ^' 

the  polyps  and  seeming  to  possess  a  phylogenetic  as  well  as  an  ontogenetic 

significance. 

The  following  were  the  most  conspicuous  phases  of  growth :  Of  the 
mesenteries  the  protocnemes  were  established  at  the  time  of  settling  of 
the  larvae,  and  the  six  pairs  are  shown  to  develop  in  a  regular  consecutive 
manner  without  any  marked  intervals  (p.  76).  Four  of  the  six  bilateral 
pairs  of  protocnemes  were  connected  with  the  stomodseum  from  the  commence- 
ment of  stationary  life,  but  at  the  termination  of  seventeen  weeks  the  fifth 
and  sixth  protocnemic  pairs  had  not  become  complete.  No  further  change 
in  the  number  of  mesenteries  took  place  for  nearly  four  weeks,  when,  in 
the  larger  polyps,  the  metacnemes  began  to  appear.  The  different  pairs  of 
these  followed  somewhat  quickly  upon  one  another  from  the  dorsal  to  the 
ventral  aspect  of  the  polyp,  until  the  complete  cycle  of  six  pairs  was  estab- 
lished (plate  3,  fig.  14).  The  polyps  remained  at  this  mesenterial  stage 
for  the  rest  of  the  time  they  were  under  observation — about  two  months. 
There  was  no  hint  of  the  appearance  of  the  third  cycle. 


64  SIDERASTREA    RADIANS. 

As  regards  the  tentacles,  six  primary  exotentacles  appeared  simulta- 
neously, shortly  after  fixation  (plate  i,  fig.  6),  followed  by  an  interruption 
of  two  or  three  weeks  before  the  entotentacles  began  to  arise.  These  were 
at  first  simple  outgrowths  of  the  disc,  and  a  long  interval  took  place  before 
a  second  moiety  appeared  in  connection  with  each  and  thus  established  the 
mature  form  (plate  2,  fig.  11).  The  longest  interruption,  however,  was  after 
the  completion  of  the  primary  twelve  tentacles,  for  in  the  most  forward  polyps 
about  three  months  elapsed  before  additional  members  appeared,  when  those 
of  another  cycle  began  to  arise  in  a  successive  manner. 

Of  the  septa  the  six  entocoelic  representatives  appeared  as  a  complete 
cycle  at  a  very  early  stage  (plate  i,  fig.  7),  followed  shortly  by  the  six  alternat- 
ing exocoelic  members ;  these  latter  were  developed  in  some  instances  as  a 
complete  cycle,  but  usually  in  successive  pairs  (plate  2,  figs.  8,  9).  Without 
any  marked  rest  other  fragments  or  nodules  were  added  to  each  system,  but 
in  such  a  way  as  to  render  it  doubtful  as  to  which  cycle  they  belonged  (plate 
2,  fig.  12).  On  the  whole,  the  growth  of  the  septa  was  more  continuous 
than  that  of  either  the  mesenteries  or  the  tentacles. 

Undoubtedly  the  most  conspicuous  interruption  for  all  the  systems  of 
organs  was  that  between  the  protocnemic  and  metacnemic  stages,  and  the 
great  differences  between  the  manner  of  appearance  of  the  two  series  of 
mesenteries,  septa,  and  tentacles  further  establishes  this  as  the  most  impor- 
tant ontogenetic  and  phylogenetic  interval  in  madreporarian  development. 
Its  significance  is  discussed  later. 

The  time  of  appearance  of  the  organs  varied  somewhat  in  different  polyps, 
sometimes  even  to  the  extent  of  several  weeks ;  but  with  regard  to  the  actual 
order  followed  no  important  differences  were  observed  in  the  mesenteries  and 
septa,  though  less  constancy  prevailed  amongst  the  tentacles.  Where  devel- 
opment was  not  in  complete  cycles  the  succession  was  bilateral,  proceeding 
from  the  dorsal  to  the  ventral  borders  of  the  polyp.  Contrary  to  what  is 
generally  assumed,  bilaterality,  not  radiality,  was  a  conspicuous  feature  of 
the  growth  from  beginning  to  end ;  and  in  most  of  the  systems  of  organs 
the  bilateral  symmetry  prevailed  for  lengthened  periods,  even  where  the 
radial  condition  was  ultimately  assumed. 

Many  of  the  young  polyps  were  preserved  at  different  stages — while  still 
adhering  to  their  original  pieces  of  glass — and  were  prepared  for  microscopic 
examination  as  a  whole.  The  process  was  as  follows :  After  narcotization 
by  means  of  magnesium  sulphate  or  menthol  the  polyps  were  killed  by 
pouring  a  solution  of  formaldehyde  over  them,  and,  still  adherent,  they  were 
transferred  to  alcohol,  afterwards  stained,  dehydrated,  cleared,  and  mounted 


POSTLARVAL  DEVKLOPMENT.  -       65 

in  Canada  balsam  as  permanent  preparations.  The  young  coralla  from 
which  the  photographs  on  plates  4-5  were  taken  were  prepared  by  macer- 
ating the  polyps  in  caustic  potash  and  afterwards  washing  or  carefully 
brushing  away  the  soft,  loosened  tissues.  After  maceration  the  skeletons 
could  generally  be  freed  from  the  glass  to  which  they  were  attached  by 
gently  tapping  the  latter,  but  in  some  cases  it  was  necessary  to  insert  the 
edge  of  a  razor  between  the  basal  plate  and  the  surface  of  the  glass. 

The  young  polyps  intended  for  sectionizing  were  detached  after  preser- 
vation by  dissolving  away  the  calcareous  deposit  with  weak  hydrochloric 
acid.  In  one  case  a  living  polyp  of  fourteen  weeks  became  detached  along 
with  the  whole  skeleton,  and,  thus  free,  continued  its  growth  for  some  time. 

In  the  course  of  their  growth  the  polyps  increased  from  i  to  2  mm.  in 
diameter,  the  oldest  polyps  reared  being  but  2  mm.  across  the  base. 

TENTACLES. 
FIRST   CYCLE    OF   KXOTENTACLKS. 

The  tentacles  began  to  make  their  appearance  two  or  three  days  after 
fixation  of  the  larvae,  as  rounded  upgrowths  over  alternate  mesenterial 
chambers.  At  this  stage  only  the  six  primary  pairs  of  mesenteries  were 
present,  giving  rise  to  six  entocoelic  and  six  exoccElic  chambers  (plate  i, 
fig.  7).  Six  tentacular  prominences  appeared  simultaneously,  equal  in  size 
and  distance  apart.  They  early  showed  an  opaque  white,  knob-like  apex 
distinct  from  a  short,  more  transparent  stem.  The  single  cycle  which  they 
constituted  served  to  delimit  the  larva  for  the  first  time  into  oral  disc  and 
column  wall,  no  indication  of  the  boundary  between  the  two  areas  being 
hitherto  determinable. 

The  stems  of  the  tentacles  were  hollow,  broad  below,  narrow  distally, 
and  perfectly  smooth,  that  is,  without  nematoblast  tubercles.  The  endoderm 
wdthin  could  be  seen  to  be  richly  supplied  with  Zooxanthellae,  while  the  ecto- 
derm was  colorless  ;  the  knob  was  solid  and  colorless,  without  Zooxanthellae, 
and  constituted  wholly  of  ectoderm  with  numerous  radiating  nematoblasts. 

At  first  the  actual  mesenteric  chambers  from  which  the  tentacles 
protruded  could  not  be  determined  with  certainty,  owing  to  the  opacity  of  the 
polyps.  Soon,  however,  as  the  polyps  grew  larger  and  the  walls  more  trans- 
parent, the  outgrowths  were  seen  to  communicate  with  the  exocoelic  chambers. 
As  noticed  below,  this  relationship  is  contrary  to  the  general  rule  of  tentac- 
ular development  hitherto  observed  among  the  Zoantharia,  and  therefore 
the  greatest  care  was  exercised  to  establish  it  beyond  a  doubt.  Bxtemally 
the  entocoelic  and  exoccelic  mesenteric  chambers  could  be  readily  determined 


66  SIDERASTREA    RADIANS. 

from  their  relation  to  the  longer  oral  axis  and  from  the  known  relation  of 
the  complete  and  incomplete  mesenteries  at  the  Edwardsia  stage  of  develop- 
ment ;  the  two  terminal  chambers  which  include  the  longer  oral  axis  are 
always  the  directive  entocoeles,  and  the  two  alternating  chambers  on  each  side 
between  these  are  the  lateral  entocoeles,  while  all  the  intermediate  divisions 
are  exocceles.  Most  of  the  young  polyps  were  so  situated  as  to  permit  of  all 
these  details  being  clearly  observed,  and  in  no  instance  were  the  first  six 
tentacles  developed  as  outgrowths  of  the  entocoeles,  but  were  always  disposed 
as  in  plates  i  and  2,  figs.  7-9. 

As  shown  in  plate  i,  fig.  7,  the  six  tentacles  first  to  appear  alternated 
with  the  first  septa,  the  latter  being  entocoelic  in  position  ;  the  smaller  septa 
which  arise  later  (plate  2,  fig.  9)  correspond  with  the  first  tentacles,  that  is, 
are  exosepta. 

FIRST    CYCLE    OF    ENTOTKNTACLES. 

A  long  interval  elapsed  between  the  appearance  of  the  primary  cycle  of 
six  exotentacles  and  the  development  of  any  others.  Toward  the  close  of 
the  fourth  week  additional  small  protuberances  of  the  disc  began  to  appear 
in  the  interspaces  between  the  primary  series  and  a  little  nearer  the  center 
of  the  disc.  Like  the  first  they  soon  showed  a  distinction  into  apical  knob 
and  stem.  In  most  polyps  the  six  new  tentacles  arose  simultaneously, 
constituting  a  second  inner  cycle  and  alternating  with  the  first ;  but  in  some 
cases  the  members  developed  successively,  though  no  dorso-ventral  or  other 
regular  sequence  could  be  established.  Any  intervals  which  occurred  in 
the  appearance  of  the  different  members  were  very  brief,  except  in  one 
instance  where  a  fortnight  passed  between  the  appearance  of  the  dorso-lateral 
and  the  ventro-lateral  pairs. 

There  were  now  twelve  tentacles  present,  arranged  in  two  alternating 
cycles  of  six  each ;  an  outer  cycle  of  six  large  exotentacles,  the  first  to  arise, 
and  an  inner  cycle  of  six  smaller  ento tentacles  appearing  later  (plate  2,  fig.  11). 

Close  examination  soon  revealed  that  the  individual  members  of  the  inner 
cycle  were  not  disposed  symmetrically  midway  in  the  interspaces  between 
those  of  the  older  cycle,  but,  as  represented  in  plate  3,  fig.  13,  were  a  little  to 
one  side.  Bven  the  tentacles  arising  from  the  axial  entocoeles  were  some- 
what to  the  one  or  the  other  side  of  the  principal  axis  of  the  polyp.  In  most 
cases  the  four  small  lateral  tentacles  were  all  situated  toward  the  dorsal 
aspects  of  their  corresponding  interspaces,  while  the  tentacles  over  the  direc- 
tive entocoeles  might  be  to  the  right  or  to  the  left  of  the  directive  axis.  This 
peculiar  arrangement  of  the  young  entotentacles  was  best  observed  when 
the  polyps  were  only  partly  extended  ;  during  full  expansion  they  appeared 


POSTLARVAL  DEVELOPMENT.  67 

almost  symmetrically  disposed  with  regard  to  the  outer  cycle  and  the  ento- 
coelic  chamber  belov/  (plate  2,  fig.  11). 

The  simple  character  of  the  entoccelic  tentacles  on  their  first  appearance 
is  in  marked  contrast  with  their  bifurcated  form  in  mature  polyps.  As 
already  mentioned  in  describing  the  adult  colony,  the  genus  Siderastrea  is 
unique  among  West  Indian  corals  in  that  its  polyps  possess  dimorphic 
tentacles.  The  entoccelic  members  consist  of  a  simple  stalk,  divided  above 
the  middle  into  two  smaller  halves,  each  of  which  is  terminated  by  a  white, 
swollen  knob ;  the  exocoelic  tentacles,  on  the  other  hand,  consist  of  a  simple 
stem  with  an  apical  knob.  It  was  also  found  that  in  nearly  mature  polyps 
the  newly  formed  entoccelic  tentacles  of  the  higher  cycles  are  sometimes 
simple,  while  the  older  members  are  double  (plate  6,  fig.  32). 

A  few  days  after  the  completion  of  the  second  (inner)  cycle  of  tentacles 
in  the  young  polyps,  the  double  character  of  its  members  began  to  assert 
itself.  From  certain  of  the  entoccelic  interspaces  another  knob-like  pro- 
tuberance arose  beside  the  first  tentacle,  which  in  time  became  a  distinct 
tentacle  with  stem  and  capitulum  like  the  other  member  (plate  2,  fig.  12)- 
The  two  moieties  thus  formed  were  for  some  time  different  in  size,  but  later 
became  equal.  The  appearance  was  as  if  two  small  tentacles  had  become 
intercalated  between,  but  a  little  central  to,  the  interspaces  of  the  first- 
formed  tentacular  cycle. 

The  second  moiety  of  the  inner  tentacles  exhibited  no  constancy  in  its 
order  of  appearance  from  the  different  entocoeles.  In  plate  3,  fig.  13,  the  ven- 
tral axial  member  has  appeared  and  also  the  two  dorso-laterals,  while  over  the 
dorsal  and  ventro-lateral  entocoeles  the  simple  tentacle  alone  is  present.  In 
four  other  polyps  the  dorsal  axial  tentacle  was  the  first  to  become  doubled, 
and  all  the  others  remained  single  for  a  few  days  longer.  In  another  polyp 
the  additional  outgrowth  first  appeared  over  the  two  axial  entocoeles. 

The  new  outgrowth  was  at  first  wholly  independent  of  the  old,  and  to  all 
appearances  the  two  were  to  be  regarded  as  distinct  tentacles,  side  by  side, 
each  with  its  own  peduncle.  Later,  however,  a  single  proximal  stem  was 
found  to  arise,  which,  uniting  the  two  as  by  a  single  stalk,  raised  them  some 
distance  above  the  disc,  and  the  organ  thus  assumed  the  form  of  the  ento- 
tentacles  already  described  in  connection  with  the  mature  colony  (plate  3, 
figs.  15,  17). 

The  adult  bifurcated  entoccelic  tentacle  is  thus  seen  to  be  primarily 
represented  by  two  distinct  tentacles,  which  only  later  are  united  by  the 
growth  of  a  common  peduncle.  It  represents  a  unique  method  of  tentacular 
growth  among  the  Anthozoa.     The  double  character  appeared  on  only  a  few 


68  SIDERASTREA    RADIANS. 

of  the  most  forward  larval  polyps,  and,  for  tlie  time  they  were  under  obser- 
vation, was  in  no  instance  established  for  all  the  six  members. 

For  another  period  of  six  weeks  or  two  months  no  further  tentacular 
increase  took  place,  though  in  the  meantime  the  second  cycle  of  mesenteries 
had  made  its  appearance. 

So  far,  four  distinct  stages  are  recognizable  in  the  development  of  the 
twelve  prototentacles  of  Siderastrea,  each  stage  separated  by  marked  intervals 
of  rest.  They  are  represented  diagrammatically  in  the  figures  a-c  on  p.  72 
as  they  would  appear  when  fully  completed. 

1.  The  stage  with  six  exocoelic  tentacles,  forming  a  single  cycle.  The 
members  are  equal  and  appear  simultaneously. 

2.  The  stage  with  twelve  simple  tentacles,  constituted  of  an  outer  cycle 
of  six  large  exotentacles  and  an  inner  alternating  cycle  of  six  small  ento- 
tentacles.  The  latter  appear  either  simultaneously  or  successively  in  an 
irregular  sequence.  They  are  not  situated  midway  over  the  mesenterial 
chambers.  The  four  lateral  members  are  disposed  a  little  towards  the  dorsal 
or  ventral  half  of  the  interspaces,  while  the  directive  members  are  somewhat 
to  one  side  or  the  other  of  the  principal  oral  axis  of  the  polyp. 

3.  Six  additional  members  of  the  inner  entoccelic  cycle  appear,  so  that 
the  cycle  is  constituted  of  twelve  independent  tentacles  in  six  pairs.  They 
arise  in  no  definite  sequence. 

4.  A  single  outgrowth  arises  over  the  middle  of  each  entoccele  and  forms 
a  common  peduncle  for  each  pair  of  entotentacles.  Thus  the  tentacular 
outgrowths  from  each  entoccele  come  to  resemble  a  bilobed  adult  tentacle. 

The  appearance  of  the  exocoelic  tentacles  in  advance  of  the  entoccelic  in 
Siderastrea  radians  is  unique  in  the  tentacular  development  of  Zoantharian 
polyps  according  to  the  researches  of  Lacaze-Duthiers,  Faurot,  von  Koch, 
Appellof  and  others,  who  have  made  us  acquainted  with  the  post-larval 
development  of  many  species  of  anemones  and  corals.  In  the  corals  Caryo- 
phyllia  cyathus^  Manicina  areolata^  and  Favia  fragum  the  twelve  proto- 
tentacles, constituting  an  inner  and  outer  cycle,  appear  simultaneously,  or 
at  most  there  is  only  a  short  interval  between  one  and  the  other ;  but 
wherever  such  an  interval  occurs,  the  inner  entoccelic  cycle  appears  in 
advance  of  the  outer  exocoelic,  and  the  entoccelic  members  are  larger  than 
the  exocoelic  (entacmasous).  In  general  it  will  be  found  that  the  production 
of  other  entoccelic  structures — tentacles  and  septa — in  corals  is  in  advance 
of  the  exocoelic. 

In  actinian  larvse  it  is  more  usual  for  eight  tentacles  to  appear  first, 
arranged  either  as  a  single  cycle  or  as  an  inner  and  outer  cycle  of  four  each. 


POSTLARVAL  DEVELOPMENT.  69 

These  correspond  witli  the  eight  interspaces  between  the  four  pairs  of 
Edwardsian  mesenteries  ;  later,  as  the  fifth  and  sixth  pairs  of  protocnemes 
increase  in  size,  other  two  pairs  of  tentacles  arise,  and  the  whole  become 
arranged  in  two  cycles  of  six  each.  Of  the  primary  eight  tentacles  in 
actinians  six  are  the  entocoelic  members,  and  they  early  assume  predomi- 
nance over  the  exocoelic  series  {e.  g.^  Lebrunia^  1899). 

Clearly  the  number  of  external  tentacles  first  appearing  is  determined 
by  the  number  of  internal  mesenterial  chambers  already  formed,  seeing  that 
the  cavities  of  the  former  are  continuations  of  the  latter.  Where,  as  in  the 
actinian  Lebrunia  (1899),  only  four  pairs  of  mesenteries  are  present  when 
the  tentacles  make  their  appearance,  it  is  manifest  that  only  eight  tentacles 
will  be  developed,  and  that  the  other  four  will  be  formed  only  when  the 
growth  of  the  fifth  and  sixth  mesenterial  pairs  has  given  rise  to  four 
additional  mesenterial  chambers.  In  the  species  of  coral  larvae  so  far  fully 
investigated  the  six  pairs  of  primary  mesenteries — four  pairs  of  macrocnemes 
and  two  pairs  of  microcnemes — are  fully  established  at  or  shortly  after  the 
time  of  fixation  and  before  the  tentacles  begin  to  protrude;  hence  six  or 
twelve  tentacles  arise  simultaneously,  one  from  each  mesenterial  chamber. 

The  long  interval  which  elapses  before  the  more  complex  entocoelic 
tentacles  are  fully  formed  may  perhaps  account  for  the  appearance  in  Side- 
rastrea  of  the  exotentacles  in  advance  of  the  entotentacles.  The  young 
polyp  after  fixation  has  to  provide  for  itself,  and  for  this  purpose  its  weapons 
of  offense  and  defense,  represented  by  the  tentacular  nematoblasts,  are 
necessary  from  the  beginning. 

The  origin  of  the  bifurcated  entotentacles  from  two  independent  moieties 
which  appear  at  different  times  and  are  later  elevated  upon  a  common 
peduncle  is  a  second  unique  feature  in  tentacular  development.  Obviously 
the  presence  of  a  double  battery  of  nematocysts  on  each  tentacle,  in  place  of 
a  single  battery,  will  be  of  advantage  to  the  polyp ;  and  the  growth  of  one 
moiety  in  advance  of  the  other  is  probably  to  be  taken  as  indicating  an 
ancestry  in  which  only  one  was  present. 

SECONDARY    EXOTENTACLES. 

Much  interest  attaches  to  the  manner  of  appearance  of  the  tentacles 
beyond  the  prototentacles,  as,  from  the  exceptional  sequence  of  the  first  two 
cycles,  it  is  impossible  to  predict  from  the  known  development  of  other  forms 
how  or  where  the  next  members  will  arise.  One  of  the  earliest  stages  is 
represented  by  the  fully  expanded  disc  in  plate  3,  fig.  15.  The  second- 
cycle  mesenteries  are  seen  stretching  along  the   disc,  decreasing   in  size 


70  SIDERASTREA    RADIANS. 

successively  from  the  dorsal  to  the  ventral  aspect.  The  six  new  pairs  of 
mesenteries  have  given  rise  to  six  additional  entocoelic  and  six  additional 
exocoelic  chambers,  from  which  further  tentacles  may  arise ;  so  that  mani- 
festly the  mesenteries  with  their  mesenterial  chambers  are  formed  in  advance 
of  their  tentacular  prolongations.  At  the  stage  represented  in  plate  3,  fig.  15, 
all  the  primary  entotentacles  are  yet  simple  except  the  two  dorso-lateral 
members,  each  of  which  consists  of  a  broad  stem  unequally  bifurcated  at 
the  dorsal  extremity.  The  dorsal  and  ventral  exotentacles  are  simple,  and 
the  broad  base  of  each  communicates  with  the  second-cycle  entoccele  below 
as  well  as  with  the  exoccele  on  each  side  of  it.  An  additional  tentacle  is 
seen  on  the  ventral  aspect  of  the  middle  primary  exotentacle  on  each  side  of 
the  polyp,  and  as  yet  they  are  smaller  than  the  exotentacles  first  formed. 
The  new  tentacle  has  pushed  the  older  a  little  to  one  side,  and  both  are 
obviously  exocoelic  in  position,  though  in  the  figure  appearing  to  extend 
over  into  the  entoccele.  There  are  now  eight  exocoelic  tentacles  forming  an 
outermost  cycle. 

A  somewhat  older  stage  is  represented  by  the  next  figure  (plate  3,  fig. 
16).  The  two  middle  exotentacles  are  equal  in  size  and  quite  free  from  one 
another,  and  both  are  now  seen  to  be  wholly  unconnected  with  the  median 
second-cycle  entocoeles.  Another  exocoelic  member  has  also  appeared  on  the 
ventral  aspect  of  the  dorso-lateral  exotentacle,  on  both  the  right  and  left  sides 
of  the  polyp,  but  as  yet  they  are  a  little  smaller  than  the  primary  exoten- 
tacles. The  four  new  exotentacles  are  situated  at  the  same  distance  from  the 
center  of  the  disc  as  the  primary  cycle  of  six  exotentacles,  and  therefore, 
instead  of  forming  a  new  cycle,  they  merely  add  to  the  number  of  members 
in  a  cycle  already  established.  The  outer  cycle  of  exotentacles  thus  consists 
of  ten  practically  equal  tentacles ;  the  two  wanting  to  complete  the  hexameral 
plan  belong  to  the  ventral  system,  which  region  is  usually  found  to  lag 
behind  the  dorsal  and  middle  systems  in  the  extent  of  its  development. 

The  two  stages  (plate  3,  figs.  15,  16)  demonstrate  the  important  fact 
that  the  primary  six  exotentacles  remain  exotentacles,  and  always  constitute 
part  of  the  outermost  cycle ;  further,  any  new  exotentacles  formed  become 
equal  in  size  with  the  primary  exotentacles  and  are  so  disposed  as  to  be 
members  of  the  same  cycle.  The  primary  exotentacles  do  not  become 
entotentacles  when  new  pairs  of  mesenteries  arise  in  the  same  radii,  as  from 
their  original  position  might  have  been  expected.  On  the  appearance  of  the 
second  cycle  of  mesenteries,  they  seem  for  a  time  as  if  communicating  with 
their  entocoeles  (plate  3,  fig.  15),  but  this  condition  is  only  temporary.  When 
additional  exotents^cles  arise  and  the  mesenteries  are  further  developed,  both 


POSTLARVAL    DEVELOPMENT.  7 1 

the  older  and  the  newer  tentacles  are  seen  to  communicate  only  with  the 
exocoeles,  and  the  second-cycle  entocceles  are  for  a  time  without  any  tentacular 
outgrowths.  The  cycle  of  exotentacles  when  complete  would  at  this  stage 
consist  of  twelve  members,  of  which  six  are  primary  and  six  are  later  forma- 
tions, the  latter  appearing  after  the  second  cycle  of  mesenteries  (fig. 
7,  d,  p.  72). 

The  tentacular  development  of  S.  radians  thus  conforms  with  the  law 
of  substitution  established  by  Lacaze-Duthiers  (1872,  '73)  and  Faurot  (1895) 
for  actinians — exotentacles  remain  exotentacles,  and  throughout  the  growth 
of  the  polyp  they  continually  change  their  relationship  to  the  entotentacles 
in  such  a  manner  as  always  to  constitute  the  outermost  cycle,  and  all  are 
uniform  in  size. 

As  in  the  case  of  the  two  cycles  of  primary  tentacles  the  secondary 
exotentacles  appear  in  advance  of  the  corresponding  secondary  entotentacles, 
so  that  now  the  cycle  of  exotentacles  contains  double  the  number  of  members 
of  the  cycle  of  entotentacles.  In  the  several  species  whose  development  was 
carried  thus  far  by  Lacaze-Duthiers  and  by  Faurot  the  exoccelic  and  entocoelic 
members  appeared  together  along  with  a  new  pair  of  mesenteries,  or  the  ento- 
tentacles were  in  advance  of  the  exotentacles  ;  later  the  entotentacles  became 
larger  than  the  exotentacles. 

SECOND  CYCLE  OF  ENTOTENTACLES  AND  THIRD  CYCLE  OF  EXOTENTACLES. 

Clearly  the  stage  beyond  that  represented  in  plate  3,  fig.  16,  will  be  one 
in  which  the  tentacles  begin  to  make  their  appearance  over  the  second-cycle 
entocceles.  Such  a  condition  is  represented  in  plate  3,  fig.  17,  taken  from  the 
same  polyp  as  plate  3,  figs.  15  and  16,  when  about  a  fortnight  older  than  the 
stage  of  plate  3,  fig.  16.  It  is  also  the  oldest  stage  reared.  From  the  middle 
second-cycle  entocoele  on  each  side  a  simple  tentacle  has  arisen,  intercalated 
a  little  beyond  the  primary  cycle  of  entotentacles,  but  yet  within  the  cycle 
of  exotentacles.  The  two  tentacles  are  the  first  representatives  of  the  second- 
cycle  tentacles  of  the  adult  polyps,  and  the  greatest  interest  attaches  to  the 
fact  that  they  have  displaced,  as  it  were,  some  of  the  members  of  the  second 
cycle  (exotentacles),  so  that  they  now  form  a  third  cycle. 

If  the  development  of  the  polyps  had  been  continued,  there  is  every 
reason  to  suppose  that  a  new  tentacle  would  have  appeared  over  each  of  the 
dorsal  second-cycle  entocceles.  Also  in  the  further  growth  of  the  two  ventral 
systems  another  exotentacle  would  have  appeared  on  each  side,  and  then  a 
tentacle  over  the  second-cycle  entocoele,  just  as  in  the  middle  systems;  then 
all  the  simple  second-cycle  entotentacles  would  in  time  have  become  bifurcated 


72 


SIDERASTREA    RADIANS. 


like  the  members  of  the  first-cycle  entotentacles.  In  general,  the  sequence 
of  the  entotentacles  would  follow  that  of  the  mesenteries  with  which  they  are 
associated,  although  in  plate  3,  fig.  17,  the  growth  within  the  middle  sextants 
is  in  advance  of  that  of  the  dorsal  sextants. 

The  completed  system  of  tentacles  would  consist  of:  (a)  Six  bifurcated 
entotentacles  constituting  the  first  cycle ;  {d)  six  alternating  bifurcated  ento- 
tentacles forming  the  second  cycle ;  (c)  twelve  alternating  exotentacles,  which 
have  appeared  at  two  wholly  distinct  periods,  making  up  the  third  cycle. 

The  developmental  relations  between  the  six  second-cycle  entotentacles 
and  the  twelve  third-cycle  exotentacles  are  somewhat  different  in  Siderastrea 


a 


d 


Fig.  7. — «r-e,  series  of  diagrams  illustrating  the  order  of  appearance  and  relationships 
of  the  first  three  cycles  of  tentacles.  The  numerals  I  and  II  indicate  the  orders 
of  entotentacles  and  X  the  exotentacles. 


from  those  hitherto  met  with  in  actinians.  Lacaze- 
Duthiers  (1872)  first  found  that  after  the  completion 
of  the  first  two  cycles  of  tentacles  (six  entotentacles 
and  six  exotentacles),  the  tentacles  of  the  third  cycle 
do  not  arise  peripherally,  alternating  with  the  mem- 
bers of  the  first  and  second  cycles,  as  would  be 
naturally  supposed,  but  that  six  new  pairs  are  insinuated  between  the  two 
primary  cycles  at  six  intervals  only.  One  member  of  each  of  the  six  new 
pairs  increases  in  size  more  than  the  other  member,  and  ultimately  consti- 
tutes the  second  cycle  of  tentacles  (entotentacles),  displacing  the  primary 
second  cycle  of  exotentacles ;  the  other  member  of  each  new  tentacular  pair 
attains  the  same  size  as  the  displaced  exotentacles,  and  along  with  them 
forms  the  third  cycle  of  twelve  tentacles.  The  third  cycle  is  thus  formed  of 
six  tentacles  from  the  original  second  cycle  and  six  tentacles  which  are  later 
formations ;  the  members  of  the  permanent  second  cycle  are  all  new  forma- 
tions which  have  displaced  the  older  second-cycle  members.  Lacaze-Duthiers 
expressed  these  relationships  under  the  term  "  law  of  substitution." 


POSTLARVAL  DEVELOPMENT.  73 

In  Siderastrea  the  second  cycle  for  a  time  consists  of  twelve  exotentacles, 
six  of  which  are  primary  formations  and  six  are  later  outgrowths.  The  latter 
appear  in  advance  of  the  corresponding  entotentacles,  not  along  with  them,  as 
seems  to  be  the  case  in  most  actinians.  Afterwards,  however,  a  new  cycle  of 
six  entotentacles  is  intercalated  between  the  original  first  and  second  cycles, 
displacing  the  latter  to  the  third  cycle,  and  itself  constituting  the  permanent 
second  cycle.  The  law  of  substitution  thus  holds  for  Siderastrea  as  for 
actinians,  though  the  relative  time  of  appearance^f  the  entotentacles  and 
exotentacles  differs  from  that  usually  followed.  Faurot  (1895)  shows  that  in 
IlyantJms  parthenopens  the  entotentacle  from  a  new  mesenterial  pair  arises 
in  advance  of  the  exotentacle,  exactly  the  reverse  of  that  in  Siderastrea. 

THIRD    CYCLE    OF    ENTOTENTACLES    AND    FOURTH    CYCLE    OF    EXOTENTACLES. 

As  the  larval  polyps  were  not  reared  beyond  the  beginning  of  the 
three-cycle  stage,  it  will  be  necessary,  in  order  to  complete  the  survey  of  the 
tentacular  development,  to  have  recourse  to  bud  polyps  of  a  colony.  From 
what  has  been  established  above  it  is  manifest  that  the  problem  is  not  to 
determine  the  manner  according  to  which  the  fourth  cycle  of  the  adult  polyp 
will  arise,  but  how  a  third  cycle  of  entotentacles  originates  and  is  inter- 
calated between  the  previously  formed  second  and  third  cycles.  The  exo- 
tentacles as  a  cycle  are  shown  to  have  no  ordinal  significance  in  studies  of 
tentacular  sequence  ;  this  importance  belongs  to  the  entotentacles.  Regarded 
cyclically  the  exotentacles  are  but  temporary  predecessors  of  the  entotentacles 
of  the  adult  polyp. 

On  any  colony  are  numerous  polyps  in  which  the  tentacles  vary  from 
twenty-four  to  nearly  forty-eight.  In  the  polyp  represented  on  plate  6,  fig. 
32,  there  are  three  third-cycle  entotentacles,  each  with  a  corresponding 
exotentacle.  The  former  are  yet  at  an  early  stage  of  development,  only  one 
moiety  of  the  tentacle  having  appeared.  In  fig.  i,  p.  12,  is  represented 
diagrammatically  the  tentacular  plan  of  the  polyp  of  which  the  mesenterial 
system  is  given  in  plate  6,  fig.  34.  Along  with  each  of  the  five  pairs  of 
third-cycle  mesenteries  (ill)  have  appeared  two  corresponding  tentacles,  and, 
comparing  with  fig.  3,  p.  26,  one  of  the  tentacles  is  seen  to  be  entocoelic  (iii) 
and  the  other  exoccelic  (x).  The  new  entotentacles,  iii,  are  situated  beyond 
the  second  cycle  of  entotentacles,  il,  but  within  the  outermost  exoccelic 
cycle ;  that  is,  they  represent  the  commencement  of  a  third  cycle  of  ento- 
tentacles, and  the  exotentacles  are  now  being  relegated  to  a  fourth  cycle. 
At  this  stage,  therefore,  some  of  the  exotentacles  may  be  considered  as 
belonging  to  the  third  cycle  and  some  to  the  fourth  cycle.     Under  the  circum- 


74  SIDERASTREA    RADIANS. 

stances  it  was  not  possible  to  say  whether  the  new  exotentacles  appeared  in 
advance  of  the  entotentacles,  but  from  the  order  actually  observed  in  the 
first  three  cycles  it  may  be  assumed  that  such  was  the  sequence.  It  will  be 
observed  that  the  new  tentacular  pairs  are  situated  on  the  dorsal  aspect  of 
the  four  sextants,  along  with  one  pair  on  the  ventral  side  of  the  right  middle 
sextant. 

New  exotentacles  and  entotentacles  have  been  found  to  arise  shortly  after 
the  appearance  of  new  mesenterial  pairs.  Therefore,  if  the  third-cycle  mesen- 
teries develop  normally  with  the  regularity  outlined  on  p.  83,  a  single  ento- 
tentacle  aud  exotentacle  would  be  expected  to  appear  on  the  dorsal  aspect  of 
each  sextant,  and  then  the  corresponding  ventral  series  would  commence, 
both  starting  with  the  dorso-lateral  systems  and  proceeding  to  the  ventro- 
lateral. In  a  colony  where  the  polyps  are  so  closely  arranged  as  in  Sideras- 
trea^  however,  the  later  growth  rarely  proceeds  according  to  such  a  regular 
law,  so  that  very  little  importance  can  be  attached  to  the  order  of  appearance 
of  the  tentacles  making  up  the  outer  cycles.  All  the  studies  on  corals  seem 
to  indicate  that  the  mesenteries,  tentacles,  and  septa  develop  conformably  to 
a  regular  plan  for  the  first  and  second  cycles,  but  that  the  regularity  is 
frequently  departed  from  in  the  later  stages. 

If  the  cyclic  hexameral  plan  of  the  tentacles  in  S.  radians  were  com- 
pleted the  formula  would  be  6,  6,  12,  24.  The  first  three  cycles  would  be 
entotentacles  and  the  last  exotentacles ;  the  members  of  any  one  cycle  would 
be  equal  in  size,  but  those  of  the  inner  cycles  somewhat  larger  than  those  of 
the  outer  cycle.  The  entotentacles  would  have  appeared  in  a  fairly  regular 
sequence,  the  inner  cycle  first,  the  middle  cycle  next,  and  the  third  cycle 
last ;  but  it  is  altogether  otherwise  with  the  fourth  cycle,  constituted  only  of 
exotentacles.  Six  of  its  members  appeared  a  little  in  advance  of  the  first 
cycle  of  entotentacles ;  other  six  a  little  in  advance  of  the  second  cycle  of 
entotentacles  ;  and  twelve  more  in  advance  of  the  third  cycle  of  entotentacles. 
The  exotentacles  at  each  stage  form  a  complete  cycle ;  first  a  cycle  of  six, 
then  a  second  cycle  of  twelve,  which  afterwards  becomes  a  third  cycle,  and, 
lastly,  a  third  cycle  of  twenty-four,  which  afterwards  becomes  a  fourth  cycle. 
In  this  way  they  always  constitute  the  last  or  outermost  cycle  for  the  time 
being.  At  the  termination  of  each  stage  the  number  of  outer  simple  exoten- 
tacles equals  the  sum  of  the  bifurcated  entotentacles  making  up  the  inner 
cycles ;  but,  until  the  adult  stage  is  reached,  as  a  cycle  they  occupy  the 
position  which  later  will  be  occupied  by  the  entotentacles. 

When,  however,  the  hexameral  cyclic  plan  is  not  complete  in  the  adult, 
that  is,  where  the  last  cycle  of  entotentacles  commenced  is  not  completed 


POSTLARVAL  DEVELOPMENT.  75 

according  to  the  hexameral  sequence,  then  the  formula  must  be  different  from 
that  given  above  if  it  has  to  express  the  true  morphological  value  of  the  tenta- 
cles as  entotentacles  and  as  exotentacles.  Cyclic  incompletion  has  been 
shown  to  be  practically  always  the  condition  in  adult  polyps  of  S.  radians ; 
two  cycles  of  entotentacles  are  hex amerously  complete,  but  the  third  cycle  of 
entotentacles  consists  of  a  variable  number,  rarely  reaching  the  required  num- 
ber twelve.  Regarded  cyclically,  some  of  the  exotentacles  belong  to  the  third 
and  some  to  the  fourth  cycles ;  but  until  they  form  a  complete  hexameral 
cycle,  it  is  preferable  to  consider  the  exotentacles  simply  as  the  outermost 
tentacles,  not  as  forming  a  cycle.  The  morphological  formula  will  thus  be 
6,  6,  X,  6  +  6  +  X,  where  x  may  be  any  number  from  one  to  twelve ;  the 
series  6,  6,  x,  will  represent  the  number  of  entotentacles,  and  6  -1-  6  +  x 
the  number  of  exotentacles.  The  number  of  exotentacles  will  vary  in  the 
same  degree  as  the  number  of  third-cycle  entotentacles.  Later,  a  similar 
formula  is  established  for  the  septa. 

The  different  tentacular  cycles  in  the  adult  polyps  of  Siderastrea  are 
much  more  widely  apart  than  in  the  larval  polyps  {cf.  plate  6,  fig.  31  ; 
plate  3,  fig.  17).  In  the  latter  the  closeness  of  the  cycles  recalls  the  condition 
characteristic  of  the  majority  of  anemones  and  corals. 

The  principal  facts  concerning  the  development  of  the  tentacles  of  6*. 
radians  may  be  now  summarized. 

1.  The  first  tentacles  to  appear  consist  of  a  cycle  of  six  members,  one 
from  each  of  the  six  primary  exocoeles.  They  arise  simultaneously,  are 
equal  in  size,  and  remain  simple  throughout. 

2.  Shortly  afterwards  six  smaller  tentacles  arise,  forming  a  second  cycle, 
situated  internally  to  the  first,  and  communicating  with  the  six  primary 
entocoeles.  They  may  develop  either  simultaneously  or  in  successive  pairs 
from  the  dorsal  to  the  ventral  aspect.  They  are  not  symmetrically  situated 
as  concerns  the  mesenterial  chambers.  Later,  another  entocoelic  tentacle 
appears  alongside  each  of  the  first  members,  and  then  a  common  peduncle  is 
formed  which  elevates  the  two  moieties  so  as  to  form  the  single  bifurcated 
tentacle  of  the  adult. 

3.  The  second  cycle  of  mesenteries  having  appeared,  a  new  exotentacle 
arises  from  each  additional  exocoelic  chamber  within  the  six  primary  systems 
of  the  polyp.  In  general  the  succession  is  from  the  dorsal  to  the  ventral 
systems,  and  the  new  tentacle  is  on  the  ventral  aspect  of  the  older  exoten- 
tacle belonging  to  the  same  system,  but  many  variations  occur.  The  six 
new  exotentacles  arrange  themselves  in  the  outermost  or  second  cycle,  which 
thus  consists  of  twelve  members,  equal  in  size,  and  all  simple. 


76  SIDERASTREA    RADIANS. 

4.  Additional  tentacles  are  now  intercalated  between  the  inner  and  outer 
cycles  already  established.  They  arise,  apparently  without  any  regular 
sequence,  from  the  six  entocoeles  of  the  second  pairs  of  mesenteries,  and,  like 
the  members  of  the  first  cycle  of  entotentacles,  are  at  first  simple,  but  after- 
wards bifurcated.  When  the  cycle  is  completed  it  constitutes  the  second 
tentacular  cycle  of  the  mature  polyp  ;  the  original  second  cycle  of  six 
exotentacles,  now  consisting  of  twelve  members,  becomes  at  this  stage  the 
outermost  or  third  cycle  (law  of  substitution). 

5.  As  the  pairs  of  third-cycle  mesenteries  develop,  two  tentacles,  one 
entocoelic  and  one  exoccelic,  arise  in  connection  with  each.  The  entocoelic 
representatives  appear  between  the  second  and  outermost  cycles  of  the  previous 
stage,  and  thus  constitute  a  new  third  cycle ;  the  exocoelic  representatives 
arrange  themselves  among  the  members  of  the  outermost  cycle,  which  now 
becomes  the  fourth  of  the  tentacular  cycles.  Here,  as  in  the  two  previous 
cycles,  the  entotentacles  are  at  first  simple,  then  another  moiety  appears,  and 
finally  a  common  stalk. 

6.  The  hexameral  cyclic  plan  of  the  third-cycle  entotentacles  in  S. 
radians  being  usually  incomplete,  the  morphological  tentacular  formula  for 
the  whole  polyp  with  four  cycles  is  6,  6,  x,  6  +  6  +  x,  where  x  may  be  any 
number  from  one  to  twelve. 

MESENTERIES. 
FIRST    CYCLE    OF    MESENTERIES    (PROTOCNEMEs) . 

The  earliest  larvae  reveal,  both  externally  and  in  sections,  only  eight 
mesenteries,  arranged  in  four  bilateral  pairs,  two  axial  and  two  lateral.  Of 
these  the  two  lateral  pairs  are  already  united  with  the  stomodseum,  but  the 
dorsal  and  ventral  axial  pairs  are  free,  the  ventral  pair  being  a  little  larger 
than  the  dorsal  (plate  i,  fig.  3).  In  other  larvae,  only  a  day  or  two  older,  the 
two  ventral  mesenteries  have  become  united  with  the  stomodaeum,  but  the 
dorsal  are  still  free,  and  two  new  pairs,  the  fifth  and  sixth,  are  beginning 
to  make  their  appearance  (plate  i,  fig.  4,  and  plate  8,  fig.  51).  At  the  stage  at 
which  the  larva  settles  four  pairs  of  mesenteries,  two  lateral  pairs  and  the 
dorsal  and  ventral  axial  pairs,  are  complete,  and  the  fifth  and  sixth  pairs  have 
become  a  little  larger,  but  remain  free  from  the  stomodseum  (plate  i,  fig.  6). 
This  is  the  stage  represented  by  all  the  larvae  at  or  shortly  after  fixation,  and 
is  evidently  one  of  importance  in  the  ontogeny  of  the  polyp,  for  no  further 
mesenterial  development  took  place  for  a  period  of  three  or  four  weeks. 

In  transverse  sections  of  the  larvae  the  vertical  muscle  fibers  of  the 
mesenteries  are  already  sufficiently  well  developed  to  permit  of  the  directive 


POSTLARVAL  DEVFXOPMENT.  77 

and  unilateral  pairs  being  established.  On  the  dorsal  and  ventral  axial 
pairs  the  musculature  is  on  the  faces  of  the  mesenteries  turned  away 
from  one  another,  which,  by  this  means,  are  recognizable  as  directives. 
The  musculature  on  each  complete  lateral  mesentery  is  on  the  ventral 
face  turned  towards  the  incomplete  mesentery,  while  this  bears  its  muscle 
fibers  on  the  dorsally  directed  face,  so  that  each  unilateral  pair  of  mesen- 
teries at  this  stage  is  made  up  of  a  complete  and  an  incomplete  moiety 
(anisocnemic). 

On  the  reasonable  assumption  that  the  comparative  sizes  of  the  mesen- 
teries indicate  their  order  of  appearance  in  the  larvae,  the  two  bilateral  pairs 
which  first  unite  with  the  stomodaeum  (plate  i,  fig.  3)  are  the  first  and  second 
pairs  to  arise.  At  the  stage  available  little  difference  in  size  is  represented 
by  these  two  pairs,  so  that  it  could  not  be  readily  determined  which  is  the 
first  pair  and  which  the  second.  The  results  of  Von  Koch  (1897),  Wilson 
(1888),  and  others  who  have  studied  earlier  stages  of  coral  larvae,  leave  no 
doubt,  however,  that  the  ventral  of  the  two  pairs  is  the  first,  and  that  the 
dorsal  pair  is  the  second.  Frequently  in  other  corals  the  first  pair  is  for  a 
time  much  better  developed  than  the  others,  and  bears  well-defined  filaments 
(1902,  p.  528).  From  the  relationships  shown  on  plate  i,  fig.  4,  and 
plate  8,  fig.  51,  there  can  be  no  uncertainty  as  to  which  are  the  third 
and  fourth  mesenterial  pairs.  The  ventral  pair  of  directives  is  larger, 
and  unites  with  the  stomodaeum  in  advance  of  the  dorsal  pair ;  it  is  there- 
fore the  third  pair  in  the  mesenterial  sequence,  and  the  dorsal  directives  will 
be  the  fourth. 

The  incomplete  mesenteries  on  plate  i,  fig.  6,  representing  the  fifth  and 
sixth  bilateral  pairs,  arise  practically  at  the  same  time,  and  for  the  most  part 
remain  equally  developed.  The  more  dorsal  of  the  two  in  some  instances 
outstrips  the  other,  and  in  other  corals  and  actinians  is  frequently  found  to 
unite  with  the  stomodaeum  in  advance  of  the  more  ventral.  Whatever  differ- 
ences exist  in  size  or  time  of  union  with  the  stomodaeum  support  the  view 
now  generally  held  that  the  incomplete  pair  of  mesenteries  between  the  first 
and  second  pairs  is  to  be  regarded  as  the  fifth,  and  hence  the  pair  between 
the  first  and  third  pairs  is  the  sixth. 

The  sequence  of  the  protocnemes  thus  established  for  Siderastrea 
radians  is  in  strict  agreement  with  that  ascertained  by  H.  V.  Wilson  (1888) 
for  Manicina  areolata^  and  by  Von  Koch  (1897)  for  Caryophyllia  cyaihus^ 
both  of  whom  had  a  complete  series  of  larval  stages  for  investigation.  It  is 
also  supported  by  the  various  early  stages  in  other  corals  which  I  have  had 
under  observation  (1902,  p.  450).     The  same  order  is  followed  by  nearly  all 


78  SIDERASTREA    RADIANS. 

actinians,*  though  the  evidence  for  the  Cerian these  and  Zoanthese  is  not  yet 
conclusive. 

The  length  of  time  the  young  polyps  of  Siderastrea  remained  with 
only  twelve  mesenteries — eight  macrocnemes  and  four  microcnemes — would 
indicate  that  the  stage  is  one  of  phylogenetic,  as  well  as  ontogenetic, 
significance.  No  marked  resting  stage  in  the  mesenterial  development 
occurred  until  this  was  reached,  while  none  of  the  polyps  showed  further 
progress  until  the  expiration  of  three  or  four  weeks,  and  some  of  the  smaller 
polyps  remained  thus  for  three  months.  Further,  the  fact  that  four  pairs 
united  with  the  stomodaeum,  while  two  pairs  remained  separate  for  the  whole 
seventeen  weeks,  the  six  pairs  of  second-cycle  mesenteries  arising  in  the 
meantime,  would  suggest  a  different  significance  for  the  complete  and  incom- 
plete groups  (8  +  4). 

The  relationship,  8  +  4,  is  characteristic  of  the  larvae  and  young  polyps 
of  Madreporaria  and  Actiniaria  generally,  and  persists  in  some  species 
throughout  the  life  of  the  polyp.  Only  the  first  four  pairs  of  mesenteries 
are  ever  united  with  the  stomodaeum  in  the  Bdwardsidae,  Gonacttma,  and 
some  other  Actiniaria,  and  in  the  mature  polyps  of  all  West  Indian  species 
of  Madrepora  and  Porites  the  same  eight  protocnemes  alone  are  complete, 
and  the  other  four  remain  incomplete. 

The  long  retention  of  freedom  of  the  fifth  and  sixth  pairs  of  protocnemes 
suggests  to  my  mind  an  ancestry  in  which  the  mesenteries  as  a  whole,  includ- 
ing the  metacnemes,  were  alternately  long  and  short,  excluding,  of  course, 
the  axial  directives.  Among  modem  examples  this  is  retained  in  the  mesen- 
terial system  of  the  zoanthids,  Porites^  and  Madrepora^  and  was  perhaps 
characteristic  of  the  Rugosa.  The  arrangement  of  the  musculature  on  the 
mesenteries  in  the  Cerianthese,  always  on  the  face  towards  one  aspect  of  the 
polyp,  can  be  also  understood  if  one  considers  that  the  incomplete  mesenteries 
present  in  such  forms  as  the  zoanthids  are  never  developed ;  the  cerianthids 
retain  the  simplest  protocnemic  stage  of  any  of  the  forms  here  considered 
having  only  four  bilateral  pairs. 

In  the  Ceriantheae  and  Zoantheae  the  mesenteries  beyond  the  protocnemes 
arise  at  only  one  or  two  restricted  regions  of  the  polyp  (bands  of  proliferation), 
not  all  round  the  circumference,  as  in  most  modem  anemones  and  coral 
polyps  ;  and  four  such  zones  or  bands  of  proliferation  would  appear  to  have 
been  characteristic  of  the  mesenterial  growth  in  Palaeozoic  coral  polyps.     The 

*  Appellof  (1900)  has  discussed  fully  the  value  to  be  assigned  the  accounts  of  Lacaze-Duthiers,  Haddon, 
and  others  as  to  various  other  sequences  of  the  primary  mesenteries  in  actinians.  Throughout  their  work 
Delage  and  H^rouard  (1902)  transpose  the  order  of  the  fifth  and  sixth  pairs  as  given  above. 


POSTLARVAL  DEVELOPMENT.  79 

mesenteries  and  septa  in  all  these  are  never  more  than  dicyclic.  With  the 
later  introduction  of  a  radial  polycyclic  arrangement  of  the  mesenteries  and 
septa,  by  the  addition  of  exocoelic  pairs  in  all  the  sextants,  the  fifth  and  sixth 
protocnemic  pairs  begin  to  unite  with  the  stomodaeum,  and  so  produce  a 
more  approximate  radial  symmetry  in  the  adult.  But  that  the  alternately 
macrocnemic  and  microcnemic  arrangement  is  more  primitive  seems  to  be 
still  suggested  in  the  long  separation  from  the  stomodseum  of  the  fifth  and 
sixth  protocnemes,  found  to  be  characteristic  of  ~  recent  actiniarian  and 
madreporarian  polyps. 

The  first  twelve  mesenteries  in  Siderastrea  have  been  shown  to  arise  in 
bilateral  pairs,  a  member  on  each  side  of  the  median  axis,  and  the  order  is 
such  that  each  new  pair  appears  alternately  in  the  successively  oldest 
chamber.  As  the  mesenteries  become  fully  established  another  paired 
arrangement  is  introduced.  The  directives  throughout  the  life  of  the  polyp 
constitute  bilateral  pairs ;  but  now  the  second  and  fifth  mesenteries  on  each 
side  form  what  I  have  termed  unilateral  pairs,  and  likewise  the  first  and 
sixth  mesenteries  on  each  side.  Though  varying  so  much  in  their  order  of 
appearance  and  in  their  primary  relations  to  one  another,  the  six  pairs  in 
the  end  constitute  a  regular  cycle  and  appear  all  of  the  same  value ;  at  first 
they  are  anisocnemic  pairs,  but  they  become  isocnemic  when  the  fifth  and 
sixth  bilateral  pairs  come  into  union  with  the  stomodaeum.  Thus  a  trul}^ 
radial  disposition  of  the  parts  results  from  a  primary  bilateral  origin,  a  result 
found  to  be  continually  recurring  in  the  development  of  the  different  systems 
of  organs. 

SKCOND  CYCLE  OF  MESENTERIES  (mETACNEMEs)  . 

For  a  period  of  several  weeks  after  fixation  no  addition  to  the  twelve 
protocnemes  took  place  ;  the  four  incomplete  members  continued  to  increase 
in  size,  though  remaining  free  from  the  stomodasum.  Mesenterial  filaments 
began  to  show  as  dense,  more  opaque  tissues  within  the  interior  of  the  polyp, 
but  their  relative  development  could  not  be  followed  in  the  living  polyp. 

Towards  the  end  of  the  fourth  week  some  of  the  polyps  presented 
rudiments  of  the  second-cycle  mesenteries.  Their  first  appearance  externally 
was  as  two  narrow  lines  along  the  column  wall,  towards  its  aboral  termina- 
tion, and  within  the  dorsal  exocoele  of  the  right  and  left  sides.  A  few  days 
afterwards  these  were  followed  by  a  similar  pair  of  lines  within  each  of  the 
two  middle  exocceles,  and  still  later  by  a  pair  within  each  ventral  exocoele 
(plate  3,  fig.  14).  The  actual  time  of  appearance  of  the  dorsal  pairs,  and 
also  the  intervals  between  the  three  sets,  varied  somewhat  in  different  polyps ; 


8o 


SIDERASTREA    RADIANS. 


the  periods  here  given  are  for  the  most  forward  specimens.  The  six  pairs 
together  constituted  the  second  cycle  of  mesenteries  or  first  cycle  of  metac- 
nemes,  the  different  pairs  diminishing  in  size  from  the  dorsal  to  the  ventral 
aspect  of  the  polyp.  Their  order  of  appearance  and  comparative  sizes  are 
diagrammatically  shown  in  fig.  8  {d-f). 

The  interval  between  the  appearance  of  the  dorsal  and  middle,  and 
between  the  middle  and  ventral  pairs,  was  usually  well  marked,  so  that  at 


Fig.  8  (<T,  b,  c,  d). — Series  of  diagrams  illustrating  the  order  of  development  of  the 
first  three  cycles  of  mesenteries,  a-c  represent  the  order  for  the  first  cycle  (protoc- 
nemes),  d,  the  first  stage  of  the  second  cycle  (metacnemes). 

any  time  the  different  phases  could  be  easily  ob- 
served. About  four  weeks  elapsed  between  the 
appearance  of  the  first  or  dorsal  pairs  and  that  of 
the  third  or  ventral  pairs.  Usually,  the  pairs  on 
opposite  sides  of  the  polyp  would  arise  simulta- 
neously, but  in  one  instance  the  left  pair  in  the 
ventral  exocoele  was  apparent  for  over  a  week  in 
advance  of  the  right  pair  in  the  corresponding 
exocoele.  In  every  instance  the  two  members  of  a 
unilateral  pair  appeared  simultaneously  and  developed  equally. 

The  metacnemes  increased  slowly  in  length,  and  reached  the  aboral 
termination  of  the  column  long  before  they  extended  to  the  distal  extremity. 
For  many  weeks  the  pairs  differed  in  their  vertical  extent  corresponding 
with  their  dorso-ventral  appearance ;  even  at  the  close  of  the  observations  the 
distinctions  were  strongly  indicated,  giving  to  the  polyp  a  bilateral  symmetry 
(plate  3,  fig.  17).  After  the  third  month  they  began  to  extend  across  the 
oral  disc,  the  dorsal  pairs  being  first  recognizable,  then  the  middle,  and  later 
the  ventral.  They  never,  however,  reached  the  stomodaeum,  this  condition 
being  also  characteristic  of  the  adult  polyp. 


POSTLARVAL    DEVELOPMENT. 


8l 


A  dorso-ventral  sequence  in  the  development  of  the  six  pairs  of  second- 
cycle  mesenteries,  as  compared  with  their  simultaneous  appearance,  has  long 
been  known  in  actinians ;  in  fact,  from  the  time  of  the  Dixons'  account  of 


Illb 


g 


Fig.  8  (*,/,  z,  h),  continued.-Series  of  diagrams  illustrating  the  order  of  deyelopment  of  the  first  three  cycles  of  mesenteries. 
*  and/-  represent  the  second  and  third  stages  for  second  cycle,  and  /  and  h  are  two  early  stages  of  third  cycle. 

the  relative  sizes  of  the  pairs  in  certain  late  larvae  of  Bunodes.  Apparently, 
however,  the  present  is  the  first  occasion  on  which  the  actual  appearance  has 
been  followed  stage  by  stage  in  the  living  polyp. 

The  developmental  stage  now  reached  is  one  which  frequently  recurs 
in  zoantharian  studies.     Of  the  protocnemes,  the  eight  Edwardsian  members 


82 


SIDERASTREA    RADIANS. 


are  complete  and  the  fifth  and  sixth  pairs  are  incomplete;  the  six  pairs 
of  second-cycle  mesenteries  are  incomplete,  and  either  eqnal  or  show  by 
differences  in  sizes  their  dorso-ventral  succession.  The  stage  may  be  found 
among  adult  Edwardsias,  though  two  or  more  pairs  of  second-cycle  mesen- 
teries may  here  be  wanting ;  adult  members  of  the  Ilyanthidae  often  present 
it,  and  probably  all  higher  actinians  in  the  course  of  their  development.  It 
is  repeated  by  the  many  pelagic  anthozoan  larvae  described  by  Van  Beneden 
(1897,  p.  189),  numbered  viii  to  xv,  and  which  the  author  regards  as  belong- 
ing to  the  Hexactiniaria. 


Fig.  8  (},k),  continued. — Series  of  diagrams  illustrating  the  order  of  development  of  the  first  three  cycles  of  mesenteries. 
j  is  a  late  stage,  and  k  the  complete  stage  in  the  development  of  the  third  cycle.  , 

A  long  interval  of  time,  and  a  fundamental  difference  in  their  manner 
of  appearance,  are  found  to  separate  the  second-cycle  mesenteries  from  those 
of  the  first  cycle.  The  primary  mesenteries  appear  in  bilateral  pairs  first 
towards  one  aspect  of  the  polyp  and  then  towards  the  other  aspect,  and  so 
on,  and  only  later  constitute  unilateral  pairs.  The  secondary  mesenteries, 
on  the  other  hand,  are  in  unilateral,  isocnemic  pairs  from  the  beginning, 
and  arise  within  the  six  primary  exoccelic  chambers  in  a  regular  dorso- 
ventral  succession,  alternating  with  the  primary  pairs.  Though  presenting 
such  a  difference  in  development,  the  two  series  agree  in  each  forming  a 
cycle  or  order  of  six  isocnemic  pairs  when  mature. 

It  was  to  emphasize  these  fundamental  differences  in  origin  between  the 
primary  mesenteries  and  the  later  members  that,  in  a  former  paper,  I  termed 


POSTLARVAL  DEVELOPMENT.  83 

the  first  cycle  of  six  pairs  of  mesenteries  "  protocnemes,"  and  all  subsequent 
mesenteries  "  metacnemes,"  whether  becoming  arranged  in  cycles  or  not. 
The  protocnemic  stage  is  practically  the  same  for  all  the  different  groups  of 
Actiniaria  and  Madreporaria.  It  is  in  the  metacnemic  stage  that  distinguish- 
ing characteristics  are  introduced,  development  in  all  cases  proceeding  in  a 
manner  altogether  different  from  that  of  the  protocnemic  stage.  The  interval 
is  thus  one  of  the  greatest  significance  in  the  phylogeny  of  the  Anthozoa,  as 
well  as  in  the  ontogeny  of  the  individual  polyp. 

The  metacnemic  development  here  established  for  Siderastrea  is  that 
most  usual  for  ordinary  anemones  and  corals  ;  new  isocnemic  pairs  appear  in 
the  six  primary  exocoeles  all  round  the  polypal  wall.  In  the  group  of  the 
Zoantheae  the  metacnemes  appear  at  only  two  restricted  regions,  one  within 
the  exocoele  on  each  side  of  the  ventral  directives,  and  each  unilateral  pair 
consists  of  a  large  and  a  small  member  (anisocnemic).  In  the  Ceriantheae 
the  metacnemes  appear  as  bilateral  pairs  at  only  one  region,  within  what 
seems  to  be  the  entocoele  of  the  ventral  directives.  Furthermore,  the  metac- 
nemes in  the  Zoantheae  and  Ceriantheae  do  not  form  one  or  more  cycles 
distinct  from  the  protocnemes  (monocyclic),  as  in  Siderastrea^  most  other 
modern  corals,  and  ordinary  anemones  (poly cyclic). 

These  differences  in  the  origin  of  the  metacnemes  separate  in  the  clearest 
manner  the  zoanthids  and  cerianthids  from  ordinary  actinians  and  corals. 
On  the  other  hand,  the  metacnemic  similarity  in  actinians  and  madreporar- 
ians  proves  that  the  two  groups  are  much  more  closely  allied  to  one  another 
than  to  any  other  group ;  the  only  important  difference  between  the  two 
consists  in  the  presence  or  absence  of  a  calcareous  skeleton. 

In  general,  it  will  be  found  that  in  the  Actiniaria  and  Madreporaria  the 
organs  as  a  whole  beyond  the  protocnemic  stage  develop  successively  from  one 
border  of  the  polyp  to  the  other.  The  adult  cyclic  arrangement  is  clearly  a 
later  modification  of  a  primary  dorso-ventral  plan  ;  in  some  instances,  however, 
the  cyclic  tendency  strikes  back,  as  it  were,  to  the  first  appearance  of  certain 
of  the  organs,  for  the  prototentacles  and  protosepta  usually  arise  a  cycle  at 
a  time.  The  second-cycle  septa  in  S.  radians  are  also  interesting  in  this 
respect,  for  in  some  pol3'ps  the  six  members  of  the  cycle  appeared  simulta- 
neously, but  in  others  in  successive  pairs  (p.  87). 

THIRD   CYCLE   OF   MESENTERIES. 

The  polyps  reared  from  the  larva  were  not  kept  alive  beyond  the  com- 
pletion of  the  first  and  second  cycles  of  mesenteries  ;  hence  for  what  follows 
as  to  the  order  of  appearance  of  the  third  cycle  recourse  will  be  had  to 


§4  SIDERASTRKA    RADIANS. 

colonial  polyps  arising  asexually  as  buds.  Fortunately,  in  any  colony, 
individuals  of  various  sizes  occur,  and  thus  all  the  stages  in  the  growth  of  the 
third  cycle  can  be  secured.  As  already  traced,  the  mesenteries  consist  of 
six  equal  pairs  of  protocnemes  all  united  with  the  stomodaeum,  the  fifth  and 
sixth  pairs  having  become  complete,  and  six  smaller,  incomplete  pairs  of 
metacnemes,  alternating  with  the  former.  Following  the  laws  of  hexactinian 
cyclic  symmetry  the  third  cycle  of  mesenteries  should  consist  of  twelve  equal 
pairs  situated  within  the  exocceles  formed  by  the  first  and  second  cycles ; 
the  problem  is  to  determine  their  order  of  appearance. 

The  earliest  stage  obtained  in  the  formation  of  the  third  cycle  of  mesen- 
teries is  diagrammatically  represented  in  fig.  8,  ^,  p.  8i,  taken  from  one 
of  the  small  bud  polyps  of  a  colony.  In  addition  to  the  primary  and  second- 
ary cycles  an  isocnemic  pair  of  mesenteries  has  appeared  on  each  side  of  the 
median  axis,  in  the  exocoele  between  the  dorsal  directives  and  the  dorsal  pair 
of  second-cycle  mesenteries.  Such  an  early  stage  would  be  expected  on  the 
dorso-ventral  succession  already  established  for  the  second  cycle.  The 
chamber  within  which  the  next  pair  of  mesenteries  will  arise  is,  however, 
one  of  much  significance.  The  succeeding  exocoelic  chamber  on  each  side 
is  between  the  dorsal  pair  of  second-cycle  mesenteries  and  the  dorso-lateral 
pair  of  first-cycle  mesenteries,  and  it  might  be  supposed  that  the  new  mesen- 
teries would  occupy  the  exocceles  in  regular  succession,  from  one  border  of 
the  polyp  to  the  other.  Instead  of  this  it  is  found  that  the  pairs  arise  succes- 
sively within  only  the  dorsal  of  the  two  exocceles  of  each  system.  This 
condition  is  shown  in  fig.  8,  ^,  p.  8i,  the  next  stage  available,  where  a  third- 
cycle  pair  (hi)  is  found  within  the  dorsal  exocoele  of  each  of  the  six  systems. 

A  later  stage  secured  in  the  establishment  of  the  twelve  third-cycle 
mesenteries  is  given  in  fig.  8,y,  p.  82,  where  an  additional  pair  (iii,  d)  has 
appeared,  this  time  within  the  ventral  exocoele  of  the  two  dorsal  systems. 
Clearly,  if  the  succession  here  indicated  were  followed  with  perfect  regu- 
larity, other  pairs  would  appear  within  the  ventral  exocceles  of  the  middle 
and  ventral  systems,  and  the  cycle  would  then  be  completed  according  to 
fig.  8,  k^  p.  82.  No  stage  exactly  corresponding  with  this  figure,  however, 
has  been  obtained,  as  the  polyps  of  S.  radians  very  rarely,  if  ever,  complete 
the  third  cycle  of  mesenteries. 

The  stages  above  presented  prove  that  in  the  establishment  of  the  third 
cycle  of  mesenteries  the  dorso-ventral  succession  is  twofold :  First,  a  series 
of  six  pairs  within  the  dorsal  exocceles,  and  then  a  similar  series  within  the 
ventral  exocceles  of  each  system. 

The  regularity  in  the  sequence,  represented  by  fig.  8,  g-k^  was  secured 


POSTLARVAL  DEVELOPMENT.  85 

only  after  examination  of  a  number  of  polyps.  In  a  colony  in  which  the 
polyps  are  so  closely  arranged  as  in  S.  radians  it  is  found  that  the  individuals 
rarely  undergo  their  later  development  with  perfect  regularity  all  round — 
some  regions  will  be  in  advance  of  the  normal  sequence  and  others  behind. 
The  polygonal  form  assumed  by  the  adults  is  evidence  that  pressure  is 
exerted  upon  a  form  which  otherwise  would  be  circular,  as  in  simple  polyps 
reared  from  larvae.  Spatial  difficulties  may  therefore  be  held  sufficient  to 
account  in  a  large  degree  for  the  many  irregularities  obtained  in  the  estab- 
lishment of  the  third  mesenterial  cycle. 

The  mesenterial  plan  of  two  other  polyps  is  given  in  figs.  3  (p.  26)  and 
1 1  (p.  100),  and  illustrates  the  variability  encountered.  In  fig.  3  the  sequence 
is  normal  except  that  a  pair  of  mesenteries  (iii^)  has  appeared  within  the 
ventral  exoccelic  chamber  of  the  right  middle  system  in  advance  of  the  pairs 
in  the  dorsal  exocoeles  of  the  ventral  systems.  The  polyp  represented  in 
fig.  1 1  presents  many  departures  from  the  normal  regularity ;  two  third-cycle 
pairs  occur  within  three  systems,  one  pair  within  another,  and  two  systems 
are  without  any  third-cycle  pairs. 

In  Astrangia  solitaria  and  Phyllangia  americana^  where  the  polyps 
are  practically  separated  from  one  another  and  retain  their  cylindrical  form 
throughout,  the  regularity  of  development  all  round  is  more  pronounced, 
and  the  general  order  of  appearance  of  the  mesenteries  established  in  S, 
radians  is  found  to  be  maintained  from  beginning  to  end  (1902,  p.  459). 

Hitherto,  the  development  of  the  third-cycle  mesenteries  has  not  been 
actually  followed  either  in  actinians  or  corals.  Faurot's  studies  in  1895  were 
confined  mostly  to  the  tentacles.  Carlgren*  has  described  a  condition  of  the 
mesenteries  in  the  actinian  Condylactis  cruetttata^  in  which  the  twelve  pairs 
of  third-cycle  mesenteries  as  a  whole,  as  well  as  the  exoccelic  chambers  in 
which  they  are  situated,  show  a  gradual  decrease  in  size  in  passing  from  the 
dorsal  to  the  ventral  border  of  the  polyp.  Pairs  \\\a  and  iwd  in  the  enumera- 
tion of  fig.  8,  k^  p.  82,  were  larger  than  pairs  \\\b  and  iii^,  and  these  than 
pairs  iwc  and  iii/;  hence,  if  the  condition  obtained  by  Carlgren  really  repre- 
sents the  sequence  followed  in  the  growth  of  the  third-cycle  mesenteries  in 
Condylactis^  it  is  altogether  different  from  that  of  corals,  being  simple  instead 
of  twofold. 

I  believe  it  will  be  found  in  corals  generally  that  the  sequence  of  the 
later  mesenteries  is  by  no  means  so  regular  as  that  of  the  earlier  cycles. 
The  order  followed  by  the  organs  in  the  first  and  second  cycles  is  fairly  con- 
stant, but  this  can  not  be  asserted  of  the  third  cycle,  and  probably  the  regularity 

*  "Zur  Mesenterienentwicklung  der  Aktinien,"  Ofvers  af  R.  vet.-Akad.  Forh.,  Stockholm,  1897. 


86  SIDERASTREA    RADIANS. 

will  be  even  less  in  still  higher  cycles.  As  the  polyps  increase  in  size  the 
forces  of  growth  are  less  likely  to  act  with  the  regularity  and  uniformity 
which  they  do  in  the  earlier  stages  when  the  polyps  are  small ;  even  though 
in  the  end  the  cycles  obtain  hexameral  completion  it  will  be  brought  about 
with  much  individual  variation. 

No  polyps  of  S.  radians  having  mesenteries  belonging  to  a  fourth  cycle 
have  been  found,  and  nothing  is  yet  known  as  to  the  normal  sequence 
according  to  which  the  cycle  is  established  in  other  species. 

The  chief  facts  concerning  the  mesenterial  sequence  in  S.  radians  may 
be  now  summarized : 

1.  The  six  pairs  of  first-cycle  mesenteries  (protocnemes)  arise  as  bilat- 
eral pairs  in  a  regular  alternation  from  one  aspect  of  the  polyp  to  the  other. 
The  first  four  pairs  early  unite  with  the  stomodseum,  but  the  two  last  pairs 
(v,  vi)  remain  free  for  a  long  period.  Later,  the  second  and  fifth,  and  the 
first  and  sixth  mesenteries,  on  each  side,  form  isocnemic  pairs,  and  the  third 
and  fourth  pairs  constitute  the  directives. 

2.  The  six  pairs  of  second-cycle  mesenteries  arise  bilaterally  as  uni- 
lateral isocnemic  pairs  on  each  side  of  the  polyp,  and  appear  successively 
in  the  primary  exocoeles  from  the  dorsal  to  the  ventral  border ;  ultimately 
they  become  equal  and  exhibit  perfect  radial  symmetry. 

3:  The  twelve  pairs  of  third-cycle  mesenteries  also  arise  bilaterally  as 
unilateral  isocnemic  pairs  on  each  side.  Normally  six  pairs  appear  in  a 
successive  manner  from  the  dorsal  to  the  ventral  aspect  of  the  polyp,  a  pair 
within  the  dorsal  exoccele  of  each  sextant ;  then  other  six  pairs  appear  in 
the  same  succession,  a  pair  within  the  ventral  exocoele  of  each  sextant. 
Generally  some  of  the  pairs  of  third-cycle  mesenteries  are  wanting  in  mature 
polyps. 

CORALLUM. 

FIRST   CYCLE   OF   ENTOSEPTA   AND    SECOND   CYCLE   OF  EXOSEPTA. 

Three  or  four  days  after  fixation  of  the  larva  the  skeleton  was  first 
observed  through  the  transparent  tissues  of  the  living  polyp  in  the  form  of 
six  small  radiating  septal  upgrowths,  practically  equal  in  size.  At  the 
same  time  a  narrow  peripheral  calcareous  ring  was  seen,  its  outer  surface 
uncovered  by  the  polypal  tissues  (plate  i,  fig.  7).  The  six  septa  were  per- 
fectly free  from  one  another  and  from  the  outer  annulus,  and  arranged  at 
equal  distances  apart  within  the  six  entoccelic  chambers,  thus  alternating 
with  the  cycle  of  six  exocoelic  tentacles  first  to  arise.  Each  septum  appeared 
as  a  somewhat  spindle-shaped  bar  with  the  upper  edge  strongly  spinous  and 
the  lower  edge  flat  and  adherent  to  the  glass  to  which  the  polyp  was  affixed. 


POSTLARVAL  DEVELOPMENT.  S7 

A  day  or  two  after  the  formation  of  the  first  cycle  of  entosepta,  the  six 
exoccelic  septa  began  to  make  their  appearance,  in  some  cases  simultane- 
ously, but  in  others  in  successive  bilateral  pairs  from  the  dorsal  to  the 
ventral  border  of  the  polyp  (plate  2,  figs.  8,  9).  Fig.  8  shows  that  a  pair 
of  septa  has  appeared  within  the  dorso-lateral  exocoeles,  mere  rudiments  of 
septa  are  found  in  the  middle  exocoeles,  and  as  yet  there  is  no  indication  of 
the  ventro-lateral  exoccelic  pair.  For  a  long  time,  as  shown  by  the  coralla 
on  plates  4  and  5,  the  dorso-lateral  pair  is  better  developed  than  the  middle 
pair,  and  the  middle  than  the  ventro-lateral  pair.  The  ventral  pair  in  nearly 
all  cases  remained  conspicuously  smaller  than  the  other  pairs.  Where,  how- 
ever, the  second  cycle  is  fully  developed  (fig.  9),  its  six  members  are  disposed 
midway  in  the  six  interspaces  between  the  members  of  the  primary  cycle, 
and  remain  shorter  than  the  latter.  As  in  the  entosepta  the  surface  is  spinous 
along  the  edge  and  over  both  lateral  faces. 

Thus,  within  the  first  week  two  complete  cycles  of  septa  (protosepta) 
were  developed — a  primary  cycle,  consisting  of  six  equal  entosepta,  and  a 
secondary  cycle  of  six  smaller  exosepta,  the  latter  having  appeared  later  and 
diminishing  in  size  from  the  dorsal  to  the  ventral  border.  A  narrow  periph- 
eral calcareous  ring,  unconnected  with  the  septa,  was  also  formed  at  the 
same  time.  This  is  shown  by  later  observations  to  be  a  marginal  continuation 
or  upgrowth  of  the  basal  plate  (p.  115),  and  therefore  to  be  regarded  as  an 
epitheca.  Only  the  Bdwardsian  mesenteries  were  united  with  the  stomodasum, 
and  of  the  tentacles  the  six  exoccelic  members  alone  were  developed. 

The  order  of  appearance  of  the  twelve  protosepta  is  thus  in  marked  con- 
trast with  that  of  the  twelve  protocnemes  with  which  they  are  associated. 
The  latter  have  been  found  to  arise  in  bilateral  pairs,  first  towards  one  aspect 
of  the  polyp  and  then  towards  the  other,  and  the  six  pairs  (four  macrocnemic 
and  two  microcnemic)  are  fully  established  before  any  of  the  septa  arise. 
The  six  entosepta,  on  the  other  hand,  appear  simultaneously ;  and  such  is 
usually  the  case  with  the  six  exosepta  in  other  corals,  though  not  in 
Siderastrea. 

The  symmetrical  growth  of  the  skeletal  structures,  represented  on  plate 
2,  figs.  9  and  10,  generally  took  place  only  in  completely  isolated  polyps, 
free  to  develop  equally  all  round,  and  even  in  these  irregularities  were  some- 
times introduced.  Among  the  young  polyps  forming  the  aggregated  minia- 
ture colony  in  fig.  5,  p.  60,  the  septal  development  was  scarcely  alike  in 
any  two.  Where,  as  at  the  two  extremities  of  the  colony,  one  polyp  partly 
overfolds  another,  only  half  the  number  of  septa  occurs,  while  in  the  others 
the  alternation  of  large  and  small  septa  is  inconstant ;  further,  the  epithecal 


88 


SIDERASTREA    RADIANS. 


wall  of  two  contiguous  polyps  is  common  along  the  lines  of  adherence,  and 
the  outline,  instead  of  being  circular,  becomes  more  or  less  polygonal  as  a 
result  of  the  mutual  pressure. 

The  protoseptal  stage  was  completed  in  nearly  all  cases  within  the  first 


Fig.  9  (a-*).— Series  of  diagrammatic  figures  illustrating  the  manner  of  development  of  the  septa  from  one  to  three  cycle* 
{cf.  plates  1-5).    The  epitheca  is  represented  as  a  white  circle  distinct  from  the  septa. 


fortnight,  and  some  of  the  smaller  polyps  never  reached  beyond  ;  indeed,  in 
a  few  examples,  the  exocoelic  cycle  was  never  completed.  Much  variation 
was  observable  as  to  the  rate  at  which  the  corallum  was  laid  down,  growth 
in  larger  polyps  being  always  in  advance  of  that  in  smaller  individuals. 


POSTLARVAL  DEVELOPMENT. 


89 


Within  the  larger,  more  vigorous  polyps,  further  calcareous  upgrowths 
began  to  be  formed  peripherally  during  the  course  of  the  third  week,  some 
appearing  as  angulated  extensions  of  the  primary  septa  and  others  wholly 
independent  of  them  (plate  2,  fig.  12).     Some  of  the  isolated  skeletal  elements 


Fig.  9  K/-j),  continued. — Series  of  diagrammatic  figures  illustrating  tlie  manner  of  development  of  the  septa  from  one  to  three 
cycles  (cf.  plates  1-5).     The  epitheca  is  shown  uniting  the  peripheral  ends  of  the  septa, 

seemed  like  short  additional  septa,  situated  on  different  radii  from  the  septa 
first  formed.  The  new  members  either  retained  their  independence  for  a 
long  period  or  early  became  fused  with  the  original  septum  in  whose  inter- 
space they  had  appeared.     In  this  latter  instance  the  original  septum  was 


90  SIDERASTREA    RADIANS. 

distinctly  angulated  at  its  peripheral  end,  and  wliere  two  additional  elements 
were  introduced  in  each  chamber  the  periphery  of  the  septum  was  strongly 
bifurcated.  In  some  few  cases  no  separate  fragments  w^hatever  would  arise 
within  an  interspace.  The  septum  would  then  retain  its  original  bar-like 
form,  but  become  longer  and  usually  more  rugged  in  outline,  showing  that 
skeletal  matter  was  being  added.  The  deposition  of  new  matter  was  usually 
more  forward  in  the  axial  chambers  than  in  the  lateral,  but  scarcely  any 
two  polyps  were  alike  in  the  detailed  appearance  assumed  by  the  skeleton 
at  this  period. 

Figs.  12, 19,  and  20,  on  plates  2  and  4,  represent  the  actual  polyp  or  coral- 
lum  during  this  stage,  while  fig.  9,  d^  p.  88,  is  an  attempt  to  represent  diagram- 
matically  the  septal  conditions  at  its  close.  It  is  a  well-defined  phase,  and 
important  as  showing  the  diflferent  methods  by  which  the  septa  may  increase 
in  length  and  complexity.  Although,  as  above  remarked,  no  two  polyps  were 
exactly  the  same,  a  general  plan  was  determinable  throughout  the  many 
examples  studied.  The  septa  in  all  but  the  pair  of  ventral  exosepta  con- 
sist of  a  simple,  more  central  portion  and  the  peripheral  additions.  The 
former  represents  the  enlarged  primary  septa  seen  in  fig.  9,  r,  p.  %^^  while 
the  latter  are  newer  formations.  In  general  the  peripheral  deposits  consist 
of  two  or  more  separate  fragments,  placed  at  an  angle  with  the  central  bar, 
the  angle  being  greater  in  the  exosepta  than  in  the  entosepta  ;  the  entocoelic 
additions  are,  in  fact,  nearly  parallel  with  the  primary  entoseptum,  and  thus 
more  strictly  radial.  The  two  axial  or  directive  septa  .are  usually  the  most 
strongly  developed  of  the  entire  series,  and  remained  thus  to  the  end ;  in 
addition,  the  dorsal  and  the  ventral  frequently  differ  much  in  form  from 
one  another. 

In  all  the  figures  it  will  be  seen  that  the  two  ventrolateral  exosepta 
remain  simple,  enlarging  but  little  beyond  their  primary  condition,  and 
the  middle  exosepta  are  likewise  somewhat  less  developed  than  the  dorso- 
lateral. Thus  a  decided  dorso-ventrality  is  indicated  in  the  growth  of  the 
peripheral  elements  of  the  skeleton,  as  was  the  case  with  the  simple  exosepta. 

For  many  weeks  afterwards  the  only  alteration  in  the  septa  consisted 
in  increased  growth  along  the  plan  thus  laid  down.  As  the  peripheral  frag- 
ments enlarged,  the  members  of  any  group  became  fused  with  one  another. 
After  the  second  month  the  development  of  the  skeleton  within  the  living 
polyp  could  with  difficulty  be  followed,  owing  to  the  complexity  of  the 
internal  tissues.  Therefore,  from  this  stage  onward  the  septa  will  be  studied 
mainly  from  macerated  corallites. 

Fig.  23,  plate  4,  is  from  a  photograph  of  the  skeleton  of  a  polyp  ten 


POSTLARVAL    DEVELOPMB:nT.  9 1 

weeks  old,  iu  which  only  six  pairs  of  primary  mesenteries  and  two  cycles  of 
tentacles  were  present,  while  fig.  9,  e^  p.  88,  is  its  diagrammatic  representa- 
tion. Bach  septum  now  bears  a  closer  resemblance  to  the  septa  in  the 
mature  cor  alii  te,  being  narrow  centrally  and  broad  peripherally,  with  spinous 
projections  over  the  entire  surface.  The  general  relationships  of  the  septa 
to  the  mesenterial  pairs  could  be  made  out  before  maceration  ;  hence  there  is 
no  uncertainty  as  to  the  orientation  of  the  corallum.  The  dorsal  and  ventral 
directive  entosepta  present  a  somewhat  bifurcated  peripheral  extremity,  the 
two  limbs  being  nearly  parallel,  but  the  four  lateral  entosepta  are  simple 
bars,  with  thickened  peripheral  ends  formed  from  the  enlargement  and 
complete  ifusion  of  the  originally  separate  elements.  On  the  other  hand, 
the  dorso-lateral  and  middle  pairs  of  exosepta  are  strongly  bifurcated  periph- 
erally, while  the  membersof  the  ventral  pair  retain  their  simple  character,  and 
are  by  far  the  smallest  of  the  six  pairs.  The  peripheral  limbs  in  the  bifurcated 
exosepta  may  be  quite  separate  from  the  single  radial  piece,  though  usually 
they  are  joined.  It  is  readily  seen  how  by  the  enlargement  and  fusion  of  the 
many  detached  fragments  within  each  mesenterial  space  of  plate  2,  fig.  12, 
such  a  septal  condition  as  that  of  plate  4,  fig.  23,  has  been  obtained. 

With  the  increase  in  thickness  of  the  septa  the  interseptal  loculi  are 
correspondingly  diminished,  but  no  actual  synapticular  unions  are  yet  formed 
across  the  interspaces.  The  dorso-lateral  exosepta  bend  laterally  and  nearly 
fuse  centrally  with  the  dorsal  directive  septum ;  the  middle  exoseptum  on 
each  side  approaches  the  corresponding  dorso-lateral  entoseptum  and  fuses 
with  it,  while  the  small  ventro-lateral  exoseptum  on  each  side  is  fused  with 
the  ventro-lateral  entoseptum  (fig.  9,  <?,  p.  88). 

Much  older  corallites,  but  still  at  nearly  the  same  stage  of  development, 
are  shown  on  plate  4  (fig.  24)  and  on  plate  5  (figs.  25-27),  and  diagrammati- 
cally  by  fig.  9,/,  p.  89.  The  dorso-lateral  exosepta  are  now  fused  with  the 
dorsal  directive  septum,  the  middle  exosepta  with  two  dorso-lateral  entosepta, 
and  the  ventro-lateral  exosepta  with  the  ventro-lateral  entosepta.  The  exo- 
septa, generally,  are  not  so  strongly  bifurcated  as  in  the  corallite  on  plate  4, 
fig.  23  (fig.  9,  <?,  p.  88).  Fig.  10,  a-d^  on  p.  96,  shows  the  diagrammatic  rela- 
tionships of  the  septa  to  the  mesenteries  throughout  these  early  stages. 

The  general  impression  produced  by  the  corallum  at  this  stage,  as  seen 
under  a  simple  lens  or  low  power  of  the  microscope,  is  that  of  two  alternating 
cycles  of  septa — a  larger  and  a  smaller.  The  tendency  to  fusion  of  adjacent 
septa,  exosepta  with  entosepta,  which  was  found  to  be  so  marked  a  feature 
of  the  adult  corallite,  is  already  exhibited  by  the  two  cycles,  and  gives  a 
bilateral  symmetry  to  the  calice. 


92  SIDERASTREA    RADIANS. 

Four  distinct  stages  in  the  development  of  the  protosepta  of  Siderastrea 
radians  are  thus  recognizable  : 

1.  The  simultaneous  appearance  of  six  equal  entosepta,  a  few  days  after 
the  larva  settles.  A  very  narrow  epitheca,  distinct  from  the  septa,  appears 
about  the  same  time. 

2.  The  appearance  several  days  later,  either  simultaneously  or  in  a 
dorso-ventral  sequence,  of  six  exosepta  which  are  smaller  than  the  entosepta 
and  alternate  with  them.     The  septa  of  both  cycles  are  simple,  spinous 
wedge-shaped  upgrowths  from  the  basal  plate. 

3.  The  appearance  towards  the  periphery  of  most  of  the  septa  of  one  or 
more  short  septum-like  bars  or  skeletal  nodules. 

4.  The  fusion  of  these  detached  fragments  with  the  main  septa,  so  as 
to  give  rise  to  a  broad  peripheral  termination  which  may  be  either  simple 
or  bifurcated ;  also  the  fusion  of  the  exosepta  with  the  entosepta  by  their 
inner  extremity. 

A  distinct  dorso-ventrality  in  the  rate  of  growth  is  maintained  from  the 
second  stage  onwards,  particularly  with  regard  to  the  exosepta,  thereby  giving 
a  bilateral  symmetry  to  the  calice. 

The  protoseptal  development  of  Siderastrea  presents  a  general  agree- 
ment with  that  of  other  corals  whose  early  history  has  been  followed. 
Lacaze-Duthiers  (1873,  ^^97),  however,  found  that  in  Astroides  calycularis^ 
Balanophyllia  regia^  Leptopsammia^  and  Cladopsammia  the  six  exosepta 
appeared  simultaneously  with  the  six  entosepta ;  but  in  Caryophyllia  cyathus 
and  others  there  is  an  interval  between  the  two  cycles  as  in  Siderastrea^ 
where;as  in  Manicina  areolata^  as  I  have  shown  (1902,  p.  491),  it  appears 
to  be  doubtful  whether  exosepta  ever  appear.  Lacaze-Duthiers  represents  a 
a  decided  bilateral  condition  in  the  early  development  of  the  skeleton  in 
Astroides  (1873,  plate  xiv,  fig.  29),  but  in  other  known  cases  the  septa  of 
each  cycle  appear  simultaneously,  and  are  equal  from  the  beginning. 

The  simultaneous  appearance  of  the  members  of  one  or  both  cycles  of 
protosepta  and  also  of  the  prototentacles  may  be  compared  with  the  succes- 
sive appearance  of  the  pairs  of  protocnemes.  The  protosepta  and  prototen- 
tacles resemble  one  another  in  that  both  appear  a  cycle  at  a  time,  and  from 
the  beginning  exhibit  radial  symmetry,  whereas  the  protocnemes  arise  in 
bilateral  pairs  according  to  a  well-defined  succession,  and,  for  a  time,  display 
a  strong  dorso-ventrality.  The  simultaneous  appearance  of  all  the  members 
of  a  cycle  is  maintained  for  the  septa  only  so  far  as  the  first  cycle  or,  at  most, 
the  second  cycle  of  the  protosepta ;  in  the  later  growth  of  the  organs  a  dorso- 
ventral  sequence  is  followed,  quite  as  conspicuous  as  that  of  the  mesenteries. 


POSTLARVAL  DEVELOPMENT.  93 

When  it  is  recalled  that  the  twelve  primary  mesenteries  and  their  cham- 
bers are  fully  established  before  the  tentacles  and  septa  begin  to  make  their 
appearance  it  can  be  understood  how  these  latter  organs  may  arise  a  cycle 
at  a  time.  In  their  development  beyond  the  protocnemic  stage  the  mesen- 
teries, tentacles,  and  septa  follow  one  another  very  closely,  and  such  would 
probably  be  the  case  were  the  different  pairs  of  protocnemes  closely  succeeded 
by  their  tentacles  and  septa.*  The  organs  as  a  whole  in  the  Zoantharia  are 
unquestionably  to  be  regarded  as  developing  in  a  bilateral  dorso-ventral 
order,  not  in  a  cyclic  manner.  Hitherto  the  polyps  in  Siderastrea  are  alone 
in  the  throwing  back  of  the  dorso-ventral ity  of  the  skeleton  as  far  as  the 
exocoelic  protosepta. 

The  forked  or  bifurcated  continuation  of  the  protosepta,  produced  either 
by  continued  peripheral  growth  or  by  the  production  of  independent  nodules, 
has  been  observed  both  in  Astroides  calycularis  and  in  Caryophvllia  cyathus^ 
in  addition  to  the  present  species.  Both  Lacaze-Duthiers  (1873,  1897)  and 
Von  Koch  (1882)  give  figures  of  the  developing  corallum  which  show  that 
the  septa  are  prolonged  peripherally  much  in  the  same  way  as  in  Siderastrea. 
Von  Koch  at  first  considered  that  the  forkings  were  concerned  in  the  for- 
mation of  the  theca  by  the  union  of  those  from  adjacent  septa,  but  in 
Caryophyllia  he  found  (1897)  the  true  theca  (Mauer)  to  arise  independently. 
In  Siderastrea  the  bifurcations  are  found  to  be  merely  peripheral  extensions 
of  the  septa,  becoming  fused  with  the  simple  septum  in  the  case  of  the 
entosepta,  but  constituting  two  new  septa  in  the  case  of  the  exosepta.  They 
in  no  way  assist  in  the  formation  of  a  theca. 

SECOND  CYCLE  OF  ENTOSEPTA  AND  THIRD  CYCLE  OF  EXOSEPTA. 

Before  describing  the  further  development  of  the  septa  it  will  be  helpful 
to  consider  what  are  the  septo-mesenterial  relationships  involved  in  passing 
from  a  polyp  with  only  two  cycles  of  septa  to  one  with  three  cycles.  In  the 
first  polyp  only  six  pairs  of  mesenteries  are  present,  and  within  the  ento- 
coeles  of  these  are  the  six  primary  entosepta ;  the  alternating  six  members 
of  the  second  cycle  of  septa  are  within  the  exocoeles,  on  radii  midway 
between  the  entosepta  (plate  2,  fig.  9).  In  the  second  polyp  with  three  cycles 
of  septa  two  alternating  cycles  of  mesenteries  are  present ;  the  first-cycle 
septa  are  contained  within  the  entocceles  of  the  first  mesenterial  cycle,  the 
second-cycle  septa  are  within  the  entocoeles  of  the  second  mesenterial  cycle, 

♦Where  in  larval  actinians  only  four  pairs  of  mesenteries  are  present  when  the  tentacles  begin  to 
develop.  oxAy  eight  of  the  latter  appear,  one  from  each  of  the  eight  mesenterial  chambers,  and  the 
other  four  necessary  to  complete  the  hexameral  plan  arise  after  the  establishment  of  the  fifth  and  sixth 
pairs  of  protocnemes. 


94  SIDERASTREA    RADIANS.- 

while  the  third-cycle  septa  are  within  the  twelve  alternating  exocoeles,  and 
are  therefore  exosepta.  Thus  at  both  stages  the  exosepta  form  the  outer- 
most cycle  {cf.  fig.  lo,  b^  f,  p.  96).  The  question  naturally  arises  as  to 
whether,  on  the  appearance  of  a  second  cycle  of  mesenteries,  the  exosepta 
of  the  first  stage,  which  there  constitute  the  second  cycle,  remain  as  the 
second  cycle  of  entosepta  of  the  later  stage ;  were  they  to  do  so  the  twelve 
exosepta  of  the  latter  would  be  the  only  new  formations.  As  regards  their 
actual  position  the  six  exosepta  of  the  early  polyp  correspond  with  the 
six  entosepta  of  the  later  polyp,  and  it  would  be  natural  to  assume  that  on 
the  appearance  of  the  second-cycle  mesenteries  the  latter  have  simply  included 
within  their  entocoeles  the  exosepta  of  a  former  stage,  and  then  new  exo- 
septa have  arisen  within  the  newly  formed  exocoeles. 

The  latter  is  the  view  commonly  held  by  writers  on  coral  development. 
Thus  Delage  &  Herouard,  in  their  "Traite  de  Zoologie  Concrete"  (1901,  p. 
558),  remark :  "  Quand,  dans  les  interloges  occupees  par  les  septes  du  dernier 
cycle,  nait  un  nouveau  cycle  de  couples  de  cloisons,  celles-ci  se  forment  de 
part  et  d'autre  du  septa  interloculaire  qui,  de  ce  fait,  devient  loculaire,  et 
bientot  un  nouveau  cycle  de  septes  se  forme  dans  les  nouvelles  interloges 
qui  viennent  d'etre  formees.  Les  cj^cles  naissent  successivement  et  jamais 
un  cycle  ne  commence  a  se  former  avant  que  le  precedent  soit  complet." 
Similarly,  J.  Stanley  Gardiner  (1902),  in  his  account  of  the  anatomy  of 
Flabellum  rubrum^  says  (p.  133,  italicizing  added) :  "As  the  growth  of  any 
corallite  proceeds,  more  and  more  septa  up  to  six  cycles  appear.  The  former 
exocoslic  order  of  septa  becofne  entocoelic  by  the  development  of  new  pairs  of 
mesenteries.  The  increase  of  mesenteries  takes  place  pari  passu  with  the 
formation  of  new  septa." 

Unfortunately,  the  relationships  involved  in  the  above  assertions  have 
not  been  actually  followed,  though  from  the  known  conditions  no  other 
arrangement  at  first  sight  seems  possible.  The  problem  is  one  of  the  most 
important  in  the  developmental  histor}^  of  corals. 

Without  doubt  the  members  of  the  primar}^  cycle  of  entosepta  in  adult 
polyps  are  the  direct  representatives  or  continuations  of  the  six  septa  first 
to  arise,  just  as  the  six  pairs  of  complete  mesenteries  throughout  the  life  of 
the  polyp  are  the  representatives  of  the  protocnemes.  Throughout  their 
later  growth  the  primary  septa  retain  their  individuality,  and  remain  within 
the  entocceles  of  the  primary  cycle  of  mesenteries.  Other  considerations, 
however,  are  involved  in  the  relationships  between  the  septa  developing  later 
and  those  of  the  adult  corallite. 

Most  of  the  polyps  of  Siderastrea  remained  for  some  time  at  the  stage 


POSTLARVAL  DEVELOPMENT.  95 

already  described  on  p.  92,  the  second-cycle  mesenteries  appearing  in  the 
meantime  and  growing  towards  both  the  basal  and  the  oral  disc.  As  indi- 
cated diagrammatically  in  fig.  10,  d^  p.  96,  the  new  mesenterial  pairs  corre- 
spond with  the  space  inclosed  by  the  bifurcations  of  the  dorso-lateral  and 
median  pairs  of  exosepta,  but  seem  as  if  about  to  embrace  the  incompletely 
developed  ventro-lateral  pair. 

About  this  time  other  calcareous  upgrowths  began  to  appear  peripherally, 
midway  within  the  exocoelic  bifurcations,  and  necessarily  inclosed  within 
the  entocceles  of  the  second-C3^cle  mesenteries.  Plate  3,  fig.  15,  gives  the 
discal  view  of  such  a  living  polyp  in  which  the  six  pairs  of  second-cycle 
mesenteries  had  been  developed  for  some  time.  The  new  mesenteries,  still 
varying  in  size  in  agreement  with  their  order  of  appearance,  have  now 
begun  to  extend  along  the  periphery  of  the  disc,  and  the  latter  is  resting 
upon  the  upper  edges  of  the  septa  with  the  tentacles  fully  expanded.  The 
septa  are  clearly  seen  through  the  transparent  disc.  Within  the  bifurcation 
of  each  dorso-lateral  and  middle  exoseptum  has  appeared  an  additional  free 
septum,  included  within  the  entocoele  of  the  second-cycle  mesenteries,  and 
in  the  same  radius  as  the  original  exoseptum.  Moreover,  the  exocoelic 
bifurcations  are  now  seen  to  be  wholly  exocoelic  in  position,  situated  in  the 
chambers  between  the  pairs  of  the  first  and  the  second-cycle  mesenteries ; 
in  the  dorso-lateral  sextants  the  forkings  are  free  from  the  original  median 
exoseptum,  but  in  the  middle  sextants  they  are  united.  The  primary  ento- 
septa  remain  simple  straight  bars,  and  such  is  yet  the  condition  of  the 
ventro-lateral  exosepta.  Fig.  10,  e^  p.  96,  is  a  diagrammatic  representation 
of  the  polyp  and  corallum  at  this  stage. 

The  new  formations  within  the  second-cycle  entocoeles  suggest  an  inde- 
pendent series  of  septa,  and  subsequent  stages  show  that  they  represent  the 
second-cycle  entosepta.  The  polyp  of  plate  3,  fig.  15,  in  fact,  displays  the 
early  stages  in  the  development  of  the  second  cycle  of  entosepta  and  the 
establishment  of  a  third  cycle  of  exosepta,  and  in  such  a  manner  that  the 
actual  relationships  between  the  new  septa  and  the  second-cycle  mesenteries 
admit  of  no  misinterpretation.  The  entosepta  of  the  second  cycle  appear 
peripherally  as  separate  formations,  but  in  their  later  growth  centrally,  as 
shown  on  plate  5,  figs.  28,  29,  and  fig.  10,^  p.  96,  they  come  into  union  with 
the  primary  exosepta,  and  the  two  then  appear  as  a  single  continuous  structure. 
Further,  each  bifurcation  of  a  primary  exoseptum  forms  a  new  exoseptum, 
belonging  to  a  third  cycle,  and  may  be  either  distinct  or  united  by  its  in  turned 
edge  with  a  second-cycle  entoseptum.  ^ 


96 


SIDERASTREA    RADIANS. 


Three   important   results  in   the   development  of   the    Madreporarian 
skeleton  are  thus  gained:  (i)  The  primary  exosepta  do  not  continue  their 


Fig.  10  (j»-f). — Series  of  diagrammatic  figures  illustrating  the  relationships  of  the  mesenteries  and  septa  in  the  establishment 
of  the  first  two  cycles  of  mesenteries  and  three  cycles  of  septa. 

growth  peripherall}'^  in  a  radial  manner,  and  constitute  entosepta  by  becoming 
included  within  the  entocoeleof  the  new  second-cycle  mesenteries.     (2)  The 


POSTLARVAL  DEVELOPMENT.  97 

secondary  entosepta  are  new  formations,  arising  within  the  second-cycle 
entocoeles  independently  of  other  septa ;  the  second-cycle  mesenteries  never 
embrace  the  exoccelic  protosepta  as  such,  but  only  after  their  fusion  with 
the  new  entosepta.  (3)  The  appearance  of  the  second-cycle  entosepta  is 
in  a  dorso-ventral  manner,  not  simultaneous,  a  cycle  at  a  time. 

Other  coralla  exhibit  different  stages,  all  pointing  to  the  same  conclu- 
sion. That  shown  diagrammatically  in  fig.  9,  y^,  p.  89,  is  in  somewhat  the 
same  stage  as  fig.  10,  e.  In  the  upper  right  sextant  the  second-cycle 
entoseptum  (11)  is  yet  very  distinct  from  the  exocoelic  bifurcations,  but  an 
irregularity  in  the  rate  of  growth  is  exhibited  by  the  corresponding  left 
sextant,  in  that  no  entoseptum  has  yet  formed.  The  growth  in  the  two 
middle  sextants  is  in  advance  of  it ;  each  of  the  sextants  contains  a  primary 
exoseptum,  a  secondary  entoseptum,  and  two  third-cycle  exosepta  as  distinct 
formations.  As  before,  the  development  within  the  two  ventro-lateral  sex- 
tants lags  behind  that  of  the  middle  and  dorso-lateral  sextants.  Fig.  9,  g^ 
from  another  corallum,  also  shows  important  intermediate  stages. 

The  next  stages  available  are  the  coralla  of  the  oldest  polyps  reared,  two 
of  which  are  represented  on  plate  5,  figs.  28,  29,  and  diagrammatically  in  fig. 
9,  y,  p.  89,  and  fig.  10,/,  p.  96.  Bach  corallite  now  consists  of  three  com- 
plete orders  or  cycles  of  septa,  the  development  within  the  two  ventral 
sextants  having  reached  the  same  stage  as  that  within  the  dorsal  and  middle 
sextants.  The  first  order  contains  six  large  septa  radially  disposed,  the 
directive  septa  a  little  larger  than  the  others.  The  second  order  also 
includes  six  septa,  turned  inwardly  in  such  a  manner  that  the  two  dorsal 
fuse  with  the  dorsal  directive  septum,  the  two  middle  fuse  with  the  dorso- 
laterals of  the  first  order,  and  the  two  ventral  with  the  ventro-lateral s  of  the 
first  order.  The  third  cycle  contains  twelve  septa,  the  two  adjacent  members 
fusing  with  each  of  the  six  septa  of  the  second  order.  The  coalescence  of 
the  septa  in  this  way  imparts  a  bilateral  symmetry  to  the  calice  which  other- 
wise would  be  radial.  Further,  adjacent  septa  are  now  for  the  first  time 
joined  by  synapticula.  The  polyps  (plate  3,  fig.  17)  forming  the  coralla  bore 
two  cycles  of  mesenteries,  so  that  the  first  and  second  orders  of  septa  are 
entosepta,  while  the  members  of  the  outermost  cycle  are  exosepta. 

Previous  stages  have  demonstrated  that  the  members  of  the  second 
order  of  septa,  although  now  continuous  bars,  have  really  a  twofold  origin. 
The  peripheral  part  of  each  appeared  as  a  separate  septum  within  an  entoccele 
of  the  second-cycle  mesenteries  and,  later  continuing  its  growth  centrally, 
fused  with  an  exoseptum  which  was  originally  a  constituent  of  the  second 
cycle  of  septa,  so  that  now  the  two  appear  as  a  single  septum.     In  each 


98  SIDERASTREA    RADIANS. 

corallum  there  are  indications  still  remaining  of  the  distinctness  of  the 
peripheral  and  central  parts  of  the  entosepta,  as  in  the  ventral  sextants  of 
fig.  28,  plate  5,  where  the  entocoelic  moiety  is  still  free  from  the  exoccelic 
portion.  The  fusion  of  the  two  parts  is  a  mechanical  necessity  in  the 
process  of  growth,  seeing  that  both  are  on  the  same  radii.  It  is  manifest 
that  it  is  only  by  securing  such  intermediate  stages  that  the  true  character 
of  the  septa  at  the  mature  stage  can  be  understood.  When  the  secondary 
entosepta  have  come  into  fusion  with  the  primary  exosepta  there  remains 
no  means  of  distinguishing  their  compound  origin,  and  the  ontogenetic  and 
phylogenetic  significance  of  the  secondary  entosepta  is  obscured. 

The  twelve  exosepta  now  forming  the  tertiary  cycle  undoubtedly  arise  as 
continuations  of  the  bifurcations  of  the  six  primary  exosepta,  but  in 
many  places,  as  on  plate  5,  figs.  27  and  28,  they  still  show  a  considerable 
amount  of  distinctness.  Regarding  them  as  continuations  of  the  two  forks 
of  each  primary  exoseptum,  it  is  manifest  that  they  are  to  be  considered 
as  appearing  in  advance  of  the  entosepta  of  the  second  cycle,  a  relationship 
which  need  not  be  wondered  at,  considering  that  in  this  species  the  exo- 
tentacles  are  also  found  to  arise  in  advance  of  the  entotentacles.  Originally 
forming  the  second  cycle,  the  exosepta  now  constitute  the  third  septal  cycle, 
their  place  in  the  sequence  being  taken  by  the  new  second  cycle  of  entosepta. 

It  is  thus  manifest  that  in  the  course  of  development  of  a  coral  the 
exosepta  of  a  former  stage  do  not  become  the  entosepta  of  a  later  stage  ; 
the  latter  are  new  formations  appearing  after  the  establishment  of  the 
mesenteries  with  which  they  correspond,  and  consequently  the  mesenteries 
and  their  included  entosepta  have  the  same  ordinal  value.  The  exosepta, 
on  the  other  hand,  have  no  ordinal  value ;  they  appear  at  each  cyclic  stage, 
always  constituting  the  outermost  cycle.  They  are,  in  a  measure,  temporary 
structures,  predecessors  of  the  entosepta,  until  the  limit  of  growth  of  the 
polyp  is  reached ;  they  serve  as  integral  parts  of  the  septal  system  of  the 
coral  during  all  its  intermediate  stages,  and  are  then  overgrown  by  later 
permanent  septa.* 

•In  some  corals  the  exoseptal  predecessors  appear  to  continue  their  independent  growth  in  situ 
without  losing  their  individuality  as  skeletal  structures  in  the  central  extension  of  the  entosepta.  In 
these  cases  the  entosepta  do  not  grow  far  enough  centrally  to  fuse  completely  with  the  exosepta  already 
there.  I  believe  it  will  be  found  that  this  is  the  true  nature  oi  pali  which  are  found  in  some  corals  as 
small  septum-like  plates  in  front  of  the  larger  septa.  The  fact  that  pali  seem  not  to  occur  before  the 
primary  cycle  of  six  septa,  but  only  before  those  of  later  origin,  is  what  we  should  expect  if  this  surmise 
be  correct.  The  primary  entosepta  have  never  had  exoccelic  predecessors,  as  is  the  case  with  the  later 
entosepta.  The  pali  would  thus  represent  the  persistent  exoccelic  predecessors  of  the  entosepta  beyond 
the  primary  cycle. 


POSTLARVAL  DEVELOPMENT.  99 

The  closest  morphological  parallel  is  proved  to  exist  between  tlie  develop- 
ment of  the  septa  and  the  tentacles.  As  previously  shown,  exotentacles  are 
present  at  each  cyclic  stage,  but  a  new  cycle  of  entotentacles  intercalates 
itself  between  the  last  cycle  of  entotentacles  and  the  exotentacles,  hence  the 
latter  always  remain  as  the  outermost  cycle ;  only  the  entotentacles,  like  the 
entosepta,  have  ordinal  value.  Thus  the  law  of  substitution  first  discovered 
by  Lacaze-Duthiers  for  the  tentacles  of  hexactinians  is  found  to  hold  for  the 
septa  also. 

Belonging  to  the  soft,  fleshy  parts  of  the  polyps,  it  can  be  easily  under- 
stood how  actual  displacement  of  the  tentacles  may  be  carried  out,  but  such 
is  not  possible  with  the  hard,  rigid  skeleton.  Hence  the  process  of  substitu- 
tion must  be  conducted  in  a  different  manner  in  the  two  sets  of  structures. 
A  new  peripheral  entoseptum  arising  independently  can  not  displace  an  inner 
exoseptum  already  occupying  the  same  radius.  In  its  growth  centrally  the 
entoseptum  simply  fuses  with  the  exoseptum,  and  thenceforward  the  whole 
of  the  septum  must  be  morphologically  regarded  as  an  entoseptum. 

The  septa,  like  the  tentacles,  are  thus  shown  to  arise  in  such  a  manner 
that  it  is  impossible  to  determine  their  order  of  development  from  their  rela- 
tionships in  the  mature  polyp. 

Though  the  septo-mesenterial  relationships  will  remain  the  same,  I 
conceive  that  a  like  adult  condition  of  the  septa  may  be  reached  in  differ- 
ent ways  in  different  forms  of  corals,  as  is  the  case  with  the  tentacles. 
The  actual  method  followed  in  Siderastrea  can  by  no  means  be  assumed  to 
be  that  characteristic  of  corals  generally.  Probably  some  of  the  stages  in 
Siderastrea  might  be  better  interpreted  were  results  available  from  other 
forms ;  such,  for  instance,  as  the  significance  of  the  forking  of  the  exosepta. 
The  fact  that  in  Siderastrea  the  septa  of  one  cycle  fuse  centrally  with  the  septa 
of  the  next  inner  cycle  probably  obscures  the  problem  somewhat,  as  com- 
pared with  forms  where  the  septa  remain  altogether  free  from  one  another. 

THIRD  CYCLE  OF  ENTOSEPTA  AND  FOURTH  CYCLE  OF  EXOSEPTA. 

None  of  the  polyps  reared  from  larvae  were  kept  alive  beyond  the  for- 
mation of  the  first  three  cycles  of  septa,  which  consist  of  two  cycles  of 
entosepta  and  one  cycle  of  exosepta.  Therefore  the  development  of  the 
fourth  cycle  of  septa  must  be  studied  from  the  bud  polyps  of  a  colony. 
Fortunately,  there  are  many  polyps  available  for  such  an  investigation,  as 
in  any  stock  most  of  the  individuals  are  at  one  stage  or  another  towards  the 
establishment  of  a  complete  fourth  cycle  of  24  septa. 


lOO 


SIDERASTREA    RADIANS. 


III2 


nil 


Ills 


ni8 


When  describing  the  septo-mesenterial  relationships  in  the  mature 
polyp,  it  was  found  that  the  fourth-cycle  septa  occur  within  the  exocceles 
between  all  the  adjacent  pairs  of  mesenteries.  On  leaving  the  last  section, 
however,  the  third  cycle  was  constituted  of  exosepta ;  only  the  first  and 
second  cycles  were  entosepta ;  therefore,  as  in  the  case  of  the  secondary  and 

tertiary  cycles,  the  problem  is  first 
to  determine  the  morphological  re- 
lationship of  the  tertiary  exosepta 
of  the  developing  polyp  to  the 
tertiary  entosepta,  and  also  to  the 
quaternary  exosepta  of  the  mature 
polyp.  Do  the  tertiary  exosepta 
at  the  stage  where  only  second- 
cycle  mesenteries  are  present  be- 
come the  tertiary  entosepta  when 
the  third-cycle  mesenteries  appear, 
or  are  they  continued  as  the 
fourth-cycle  exosepta,  in  which  case 
the  tertiary  entosepta  arise  de 
novo  ? 

The  relationships  to  be  determined  have  been  studied  by  means  of 
serial  sections  of  nearly  mature  decalcified  polyps.  A  complete  series  of 
stages,  representing  the  development  of  a  pair  of  third-cycle  mesenteries 
along  with  the  associated  septa  in  their  relation  with  the  older  mesenteries 
and  septa,  is  given  on  plate  9,  figs.  54-60.  The  figures  are  taken  from  a 
series  of  transverse  sections  of  a  retracted  polyp,  and  will  be  described  as 
seen  in  passing  from  the  lower  stomodaeal  region  to  the  uppermost  margin 
of  the  calice  and  polyp.  On  account  of  the  presence  of  synapticula  and  the 
peripheral  resorption  of  the  mesenteries,  certain  complications  are  introduced 
which  render  a  clear  conception  of  the  stages  somewhat  more  difiicult  than 
would  otherwise  be  the  case.  The  diagrammatic  figures  on  p.  103  will  assist 
in  following  the  description  of  the  sections. 

The  polyp  from  which  the  drawings  were  made  is  that  of  which  the 
mesenterial  plan  is  diagrammatically  represented  in  fig.  11,  p.  100.  Within 
the  ventral  system  on  the  left  side  occurs  a  pair  of  small  third-cycle  mesenteries 
(11I3),  situated  on  the  dorsal  aspect  of  a  pair  of  second-cycle  mesenteries 
(11).  Plate  9,  figs.  54-60,  represents  the  members  of  this  ventral  system 
at  different  levels,  and  the  mesenteries  and  septa  are  indicated  by  the  same 


Fifi.  II. — Diagram  of  mesenteries  in  the  polyp  from  which  figs. 
54-60,  on  plate  9,  were  taken. 


POSTLARVAL    DEVELOPMENT.  lOI 

lettering  {a-j)  throughout.  In  the  figures  mesentery  a  is  the  lower  moiety 
of  the  ventro-lateral  pair  of  first-cycle  mesenteries,  mesentery/"  is  the  left 
moiety  of  the  ventral  directives,  mesenteries  d  and  e  are  the  pair  of  second- 
cycle  mesenteries,  while  b  and  c  are  the  two  members  of  the  rudimentary  pair 
of  third-cycle  mesenteries.  The  incompletely  represented  entoseptum  to  the 
left  of  ^  is  a  member  of  the  primary  cycle  of  septa,  as  is  also  the  partial 
septum  to  the  right  of  f\  the  entoseptum  d-e  belongs  to  the  secondary 
cycle,  while  the  septa  a-d  and  <?-/,  in  figs.  54-56  are  exosepta.  The  upper 
margin  of  the  section  in  fig.  54,  is  formed  by  the  stomodaeal  wall,  while  that 
in  all  the  subsequent  figures  is  either  the  depressed  disc  or  column  wall. 
This,  upon  retraction,  has  come  to  rest  upon  the  septal  edges,  and  adapts 
itself  to  them  in  outline,  being  thrown  into  ridges  and  furrows. 

Fig.  54  represents  a  section  of  the  sextant  taken  from  the  lower  stomodaeal 
region.  The  members  of  the  mesenterial  pair,  b^  c^  of  fig.  11,  have  not  yet 
reached  this  level,  so  that  the  sextant  contains  only  the  pair  of  incomplete 
second-cycle  mesenteries,  d^  e.  The  mesenteries  a  and  y  are  united  with  the 
stomodaeal  wall  centrally,  but  their  peripheral  extremity  has  undergone 
resorption  and,  therefore,  is  free.  At  this  level  the  second-cycle  mesenteries, 
d^  e^  are  feebly  represented,  being  much  reduced  peripherally  as  a  result  of 
resorption.  The  entoseptum  d-e  is  broad,  but  the  exosepta  a-d  and  e-f  on 
each  side  of  it  are  narrow  and  partly  turned  towards  it.  Exoseptum  a-d  is 
bifurcated  at  its  peripheral  end,  but  before  dividing  is  connected  by  synap- 
ticula  with  the  entosepta  on  each  side.  Within  the  angle  of  bifurcation 
occurs  a  small,  empty  loculus. 

Plate  9,  fig.  55,  is  from  a  section  immediately  above  the  level  of  the 
depressed  stomodaeum,  so  that  it  is  bounded  above  by  the  disc  resting  upon 
the  septal  edges.  At  this  level  mesenteries  d  and  e  are  larger  than  in  the 
former  section  and  are  without  any  peripheral  degeneration;  exoseptum  e-f\s 
united  by  its  inner  edge  with  entoseptum  d-e^  but  the  bifurcated  exoseptum 
a-d  is  free.  Within  the  loculus,  at  the  angle  of  bifurcation  of  the  exoseptum, 
the  rudiments  of  the  pair  of  mesenteries,  b^  c,  have  appeared,  but  at  this  stage 
there  is  no  septal  formation  whatever  within  their  entoccele.  Already,  there- 
fore, it  is  manifest  that  the  new  pair  of  mesenteries  does  not  inclose  a  pre- 
viously formed  exoseptum ;  the  two  members  lie  close  together  within  the 
peripheral  bifurcation  of  an  exoseptum,  a-d. 

Plate  9,  fig.  56,  is  taken  from  a  section  at  a  somewhat  higher  level. 
The  second- cycle  mesentery  d  is  now  united  with  the  depressed  disc,  but 
the  other  moiety  {e)  of  the  pair  is  still  free ;  in  none  is  there  any  peripheral 
resorption.     Exoseptum  ^-y  is  once  more  distinct  centrally  from  entoseptum 


I02  SIDERASTREA    RADIANS. 

d-e^  as  in  fig.  54  ;  a  synapticular  growth  from  the  left  limb  of  septum  a-d 
perforates  the  mesentery  b^  and  a  slight  depression  of  the  wall  between  the 
mesenteries  b  and  c  is  the  first  indication  of  an  entocoelic  skeletal  ingrowth 
about  to  separate  the  two. 

The  section  from  which  fig.  57  was  taken  reveals  important  altera- 
tions taking  place.  The  second-cycle  mesenteries  d  and  e  are  both  united 
with  the  discal  walls,  and  along  with /"are  at  this  level  not  perforated  by 
any  synapticular  bars.  The  two  peripheral  limbs  of  septum  a-d  are  now 
becoming  free  from  one  another  at  their  inner  angle,  and  also  from  the  central 
radial  portion  ;  both  may  be  regarded  as  distinct  exosepta  a-b  and  c-d^  though 
formerly  they  appeared  only  as  the  bifurcations  of  the  single  exoseptum  a-d. 
For  the  first  time  an  actual  entoseptum  now  occurs  in  the  entocoele  of  the 
new  pair  of  mesenteries  b  and  ^,  and  at  its  free  extremity  is  on  the  point  of 
uniting  with  the  synapticular  process  from  the  exoseptum  a-b.  The  septum 
b-c  is  clearly  a  new  formation,  appearing  within  the  now  widely  bifurcated 
peripheral  extremity  of  the  original  septum  a-d^  or,  better,  between  the  two 
exosepta  a-b^  c-d. 

The  septal  modifications  suggested  by  fig.  57  are  completed  in  the  next 
stage,  fig.  58.  The  exoseptum  c-d  is  now  distinct,  like  septum  a-b,  from  the 
central  radial  part,  while  exoseptum  a-b  and  entoseptum  b-c  are  united  with 
one  another  towards  their  free  edge  by  a  synapticulum  perforating  the 
mesentery  b. 

The  section  represented  by  fig.  59  passes  through  the  exsert  edges 
of  the  septa,  so  that  the  column  wall  forms  the  lower  boundary  while 
the  upper  boundary  passes  through  the  tentacular  region  of  the  disc,  four 
tentacular  thickenings  being  represented.  The  mesenteries  extend  uninter- 
ruptedly from  one  wall  to  the  other,  and  include  the  free  septal  edges  within 
their  chambers.  The  mesentery  c  is  still  free  at  its  inner  end,  while  the 
mesentery  b  is  perforated  by  the  synapticulum  still  joining  septa  a-b  and  b-c. 
The  central  portion  of  the  original  exoseptum  a-d)i2iS  now  nearly  disappeared. 

The  next  stage,  fig.  60,  is  from  a  section  near  the  uppermost  extremity 
of  the  retracted  polyp  and  its  calice,  so  that  now  each  septum  and  the  body 
wall  surrounding  it  are  becoming  distinct.  The  mesenteries  b  and  c  both 
extend  across  the  two  portions  of  the  column  wall,  and  the  septa  a-b,  b-c,  and 
c-d  are  distinct  from  one  another,  the  entoseptum  b-c  being  as  yet  smaller 
than  the  exosepta  on  each  side  of  it. 

Omitting  the  synapticula  as  merely  incidental  structures  the  develop- 
ment of  the  three  septa  a-b,  b-c,  and  c-d  C2i.ii  be  diagrammatically  represented 
as  in  fig.  12  {a-f)  below,  which  shows  also  their  relations  to  the  mesenteries. 


POSTLARVAL    DEVELOPMENT. 


103 


The  first  two  figures  {a,  b)  correspond  with  plate  9,  figs.  54-56,  of  the 
sections.  The  exoseptum  is  here  simple,  but  bifurcated  towards  its  periph- 
eral extremity.  At  first  no  mesenteries  are  included  within  the  angle  of 
bifurcation,  but  a  pair  has  appeared  in  fig.  12,  b.  In  relation  to  the  mesen- 
teries the  two  limbs  are  clearly  exocoelic,  and  may  even  at  this  stage  be 
regarded  as  two  individual  exosepta. 

In  fig.  12,  c^  one  exoseptum  has  become  distinct,  and  a  short  septum  has 
appeared  midway  between  the  two  older  septa  {cf.  plate  9,  fig.  57).  Clearly 
this  is  a  new  entoseptum  arising  a  little  later  than  the  new  pair  of  mesen- 
teries. In  fig.  12,  d,  the  right  exoseptum  has  also  separated  from  the  more 
central  portion  which  is  along  the  same  radius  as  the  entoseptum  {cf.  plate 
9,  fig.  58).  The  next  figure,  e,  shows  the  exsert  septa  (cf.  plate  9,  fig.  59); 
the  central  part  is  becoming  smaller,  while  it  has  disappeared  in  the  last 


rrm-SimfLUi 
2.  £  d.  S. 

Fig.  13. — Series  of  dtasrammatic  figures  illusirating  figs.  54-63  on  plate  9. 


figure,  /  {cf.  plate  9,  fig.  60),  The  entoseptum  throughout  extends  the 
shortest  vertical  distance  of  the  three,  and  so  far  as  the  development  has  pro- 
ceeded remains  the  smallest  radially.  Clearly  when  growth  is  completed 
the  three  septa  will  be  related  as  shown  in  the  six  groups  of  fused  septa  in 
fig.  10,/,  p.  96,  which  is  the  usual  relationship  of  the  exosepta  and  entoseptum ; 
the  entoseptum  in  its  further  growth  will  have  extended  more  centrally  and 
fused  with  the  central  portion  of  the  original  exoseptum,  and  the  two  exosepta 
will  turn  inwardly  to  unite  with  it. 

The  series  of  sections  plainly  demonstrates  that  a  new  entoseptum 
arises  shortly  after  a  new  pair  of  mesenteries,  not  in  advance  of  it,  and  the 
two  peripheral  limbs  of  a  bifurcated  exoseptum  become  distinct,  so  that  each 
constitutes  a  new  exoseptum.  The  new  entoseptum  takes  the  place  of  the 
inner  simple  part  of  an  exoseptum  situated  in  the  same  radius. 

The  members  of  the  third-cycle  entosepta  of  the  older  polyp  are  new 
formations  which  take  the  place  of  the  earlier  third-cycle  exosepta.  The 
dorsal  and  ventral  moieties  of  the  bifurcated  periphery  of  the  third-cycle  exo- 


I04  SIDERASTREA    RADIANS. 

septa  become  fourth-cycle  exosepta ;  in  other  words,  the  fourth-cycle  exosepta 
in  the  mature  polyp  are  the  peripheral  continuations  of  the  third-cycle  exo- 
septa, which  in  their  turn  have  been  shown  to  be  the  peripheral  bifurcations 
of  the  primary  exosepta.  Exosepta  thus  remain  exosepta  to  the  end,  each 
time  constituting  a  later  cycle  as  new  entosepta  arise  to  take  their  place. 

The  results  from  a  study  of  a  series  of  sections  from  a  nearly  mature 
bud  polyp  thus  agree  stage  by  stage  with  those  obtained  in  the  progressive 
development  of  the  three  cycles  of  septa  in  polyps  reared  from  larva.  The 
figures  given  bear  the  closest  comparison  with  the  corresponding  details  in 
fig.  lo  (d-f),  p.  96,  which  represents  three  stages  in  the  septal  development 
of  a  larval  polyp. 

Close  examination  with  a  lens  of  the  surface  of  macerated  coralla 
often  reveals  one  or  more  stages  similar  to  the  above.  In  practically  all 
the  corallites  of  a  colony  the  alternation  of  large  and  small  (entoccelic  and 
exoccelic)  septa  is  strongly  marked ;  but  amongst  the  youngest  corallites, 
especially  those  around  the  margin  of  colonies,  the  regularity  of  the  alter- 
nating large  and  small  septa  is  not  always  so  pronounced.  Occasionally  a 
group  of  two  or  three  septa  is  seen  presenting  quite  different  relationships, 
which  can  be  explained  only  upon  the  septal  development  here  described. 

An  exoseptum  is  sometimes  seen  with  its  peripheral  end  conspicuously 
bifurcated,  as  in  fig.  12,  «,  p.  103  ;  in  several  instances  a  stage  in  which  a 
septum  is  beginning  to  appear  midway  between  the  bifurcation  has  been 
met  with,  recalling  the  conditions  in  fig.  12,  ^,  d ;  while  very  often  in 
a  group  of  developing  septa  a  marked  interval  occurs  between  the  central 
and  the  peripheral  halves  of  an  entoseptum,  and  the  exosepta  on  each  side 
are  wholly  free.  In  these  instances  it  would  seem  that  the  new  entoseptum 
has  not  yet  fully  united  with  the  central  part  of  the  old  exoseptum,  being 
in  the  same  stage  as  fig.  12,  d^  e. 

It  still  remains  to  be  seen  what  is  the  order  followed  in  the  appearance  of 
the  third-cycle  entosepta,  for  these  do  not  appear  a  cycle  at  a  time  any  more 
than  the  six  members  of  the  second  cycle  of  entosepta.  The  septa  alone  in 
the  dried  corallum  are  insufiicient  to  enable  their  sequence  to  be  made  out,  as 
they  afford  no  certain  means  by  which  the  principal  or  directive  axis  can  be 
determined,  and  from  this  the  dorsal  and  ventral  borders  of  the  calice.  It 
has  been  shown,  however,  that  in  the  case  of  each  of  the  three  cycles  of  ento- 
septa the  mesenteries  appear  in  pairs  only  a  little  in  advance  of  the  corre- 
sponding entosepta  within  them ;  therefore,  if  the  sequence  of  the  third-cycle 
mesenteries  be  determined,  it  can  be  assumed  that  the  third-cycle  entosepta 
follow  the  same  order. 


POSTLARVAL  DEVELOPMENT. 


105 


The  order  of  appearance  of  the  twelve  pairs  of  third-cycle  mesenteries 
has  been  fully  described  on  p.  83.  It  is  found  that  the  members  arise  after 
the  formation  of  the  six  second-cycle  pairs  as  unilateral  pairs  in  a  twofold 
succession.  Normally  a  pair  appears  within  the  dorsal  exocoele  of  each 
sextant,  successively  from  the  dorsal  to  the  ventral  border  of  the  polyp  ;  then 
another  pair  appears  within  the  ventral  exocoele  of  each  sextant,  following 
the  same  succession  as  the  first  series. 


tiif    X      1      % 

Fig.  13. — Diagram  illustrating  the  order  of  developmtnt  of  the  first  three  cycles  of  septa  (I-III). 
The  small  letters  accompanying  the  Roman  numerals  indicate  the  sequence  of  the  indU 
vidual  septa  in  the  cycle. 

The  normal  sequence  of  the  third-cycle  mesenteries  in  Siderastrea 
being  established,  we  are  justified  in  assuming  that  a  like  succession  will  be 
maintained  by  the  third-cycle  entosepta,  seeing  that  they  arise  shortly  after 
the  mesenteries  with  which  they  are  associated.  This  is  shown  in  fig.  13, 
p.  105,  by  the  numerals  iii^-iiyC  The  figure  also  represents  the  normal 
sequence  for  all  the  entosepta  of  the  first  three  cycles.  The  six  primary 
septa  appear  together,  a  cycle  at  a  time  ;  the  six  members  of  the  second  order 
arise  bilaterally  in  a  simple  dorso-ventral  succession ;  the  twelve  members  of 
the  third  also  arise  bilaterally  in  a  dorso-ventral  succession,  but  in  two 
series — first,  a  series  of  six  within  the  dorsal  of  the  two  interspaces  in  each 


Io6  SIDERASTREA    RADIANS. 

sextant,  and  then  the  remaining  six  in  a  like  order,  but  within  the  ventral 
of  the  two  interspaces.  The  exosepta  constituting  the  last  outermost  cycle 
have  no  corresponding  ordinal  significance. 

Studies  on  the  mesenterial  sequence  of  other  corals  indicate  that  a  similar 
septal  succession  will  in  all  probability  be  followed  by  forms  in  which  the 
adult  calice  shows  a  regular  hexameral  cyclic  plan.  Individual  departures 
from  the  order  may  be  expected,  but  are  to  be  looked  upon  as  irregularities ; 
regularity  of  growth  of  the  higher  cycles  is  by  no  means  so  pronounced  as 
in  the  first  and  second  cycles,  which  are  less  likely  to  be  influenced  by 
spatial  considerations.  The  sequence  given  is  altogether  different  from  any- 
thing which  has  hitherto  been  surmised  for  any  coral,  and  further  studies  are 
desirable  to  determine  how  far  it  admits  of  general  application  in  the  group. 

From  what  has  been  revealed  it  is  manifest  that  the  exosepta  do  not 
possess  any  true  ordinal  sequence  comparable  with  that  of  the  entosepta. 
Exosepta  have  been  found  to  be  present  at  each  stage,  always  constituting 
the  outermost  cycle,  and  equaling  in  number  the  sum  of  the  inner  entosepta. 
We  maj''  consider  them  as  the  direct  continuations  of  the  six  primary  exo- 
septa, or,  less  likelj'',  as  arising  anew  with  each  cycle  of  entosepta.  Regarded 
as  the  persistent  representatives  of  the  primary  exosepta,  they  more  nearly 
conform  to  the  law  of  substitution  of  the  exotentacles  in  actinians  as  estab- 
lished by  Lacaze-Duthiers  and  Faurot.  In  actinians  generally  it  is  found 
that  after  the  protocnemic  stage  the  tentacles  appear  two  at  a  time,  one  ento- 
coelic  and  one  exocoelic,  corresponding  with  the  two  chambers  formed  upon 
the  appearance  of  a  new  pair  of  mesenteries ;  sometimes  the  entotentacles 
appear  in  advance  of  the  exotentacles,  the  reverse  of  what  happens  in  Side- 
rastrea  radians.  The  entotentacles  are  always  larger  than  the  exotentacles, 
the  length  of  the  former  being  in  accordance  with  the  order  of  appearance  of 
the  cycle  to  which  they  belong,  the  largest  being  the  first  to  appear.  The 
exotentacles  all  attain  an  equal  length  and  are  all  relegated  to  the  outermost 
cycle,  whatever  be  the  cycle  of  entotentacles  with  which  they  appeared.  They 
constitute  a  single  cycle  of  which  the  members  are  always  smaller  than  those 
of  the  cycle  of  entotentacles  last  to  appear,  and  the  number  of  exotentacles 
in  the  la^t  cycle  is  always  half  the  total  number  of  tentacles,  and,  of  course, 
equal  to  the  number  of  entotentacles. 

As  new  entotentacles  are  added  the  exotentacles  become  pushed  aside  so 
as  to  occupy  different  radii  at  different  times.  The  calcareous  septa  being 
hard,  fixed  structures,  do  not  admit  of  such  rearrangement ;  the  new  septal 
growth  has  to  be  adapted  to  the  old,  resulting  in  the  fusion  of  the  new  ento- 
septa with  the  old  exosepta. 


POSTLARVAL  DEVELOPMENT.  I07 

Partial  studies  on  other  corals,  as  well  as  considerations  on  the  tentacular 
development  in  actinians,  suggest  that  the  exosepta  may  arise  in  different  ways 
in  different  species,  and  that  a  more  precise  significance  as  to  their  relation- 
ships at  different  stages  ma}'"  be  forthcoming  than  is  possible  in  Siderastrea, 
There  are  indications  that  in  some  forms  an  entoseptum  and  an  exoseptum 
arise  together,  thus  more  closely  recalling  the  method  followed  by  the  tenta- 
cles. Regarded  as  arising  anew  in  each  cycle,  the  two  exosepta  in  Siderastrea 
appear  somewhat  in  advance  of  the  entoseptum  which  is  included  between  them. 

The  relationships  proved  to  exist  between  entosepta  and  exosepta  involve 
important  considerations  when  the  cyclic  hexameral  sequence  is  not  completed 
in  the  mature  corallite,  as  almost  invariably  happens  in  S.  radians^  as  well 
as  in  many  other  species  of  corals.  As  regards  both  the  septa  and  mesen- 
teries it  is  found  that  the  last  cycle  is  rarely  a  multiple  of  6,  but  some 
irregular  number  from  i  to  32,  resulting  from  the  fact  that  at  maturity  the 
polyp  does  not  complete  the  last  cycle  begun.  Bxosepta  have  been  shown 
to  appear  always  in  close  association  with  entosepta,  whatever  be  the  number 
making  up  a  corallite ;  and,  as  often  remarked,  the  two  series  are  equal  in 
number  and  the  exosepta  always  outermost  in  position. 

If  we  regard  a  septal  cycle  as  made  up  only  of  entosepta,  or  of  exosepta, 
then  in  mature  corallites  of  5.  radians  the  third  entoccelic  cycle  and  fourth 
exoccelic  cycle  of  septa  will  vary  in  the  same  degree.  Whatever  number  of 
entosepta  be  lacking  from  the  third  cycle  to  form  the  complete  cycle  of  12, 
a  like  number  of  exosepta  will  be  wanting  from  the  fourth  cycle. 

When  describing  the  number  of  septal  cycles  within  a  calice,  the  cyclic 
hexameral  plan  of  which  is  incomplete,  it  is  usual  in  systematic  works  on 
corals  to  regard  the  hexameral  multiples  as  completed  as  far  as  the  number 
of  septa  will  permit,  and  then  to  relegate  to  the  last  cycle  all  the  remaining 
septa  not  included  in  the  hexameral  formula.  The  cycles  are  all  supposed 
to  be  hexamerously  complete  with  the  exception  of  the  last.  Thus,  with 
regard  to  S.  radians^  Milne-Bdwards  states :  "  Three  cycles  of  septa  com- 
plete, and,  in  general,  a  variable  number  of  a  fourth  cycle."  Likewise  Verrill 
(1901,  p.  153),  describing  the  same  species,  says:  "They  [the  septa]  form 
three  complete  cycles,  with  part  of  the  fourth  cycle  developed,  so  that  the 
number  is  usually  36  to  40." 

The  relationships  proved  to  exist  between  the  entosepta  and  exosepta  in- 
dicate that  the  above  formulae  do  not  express  the  true  morphological  charac- 
ter of  the  septa.  Any  hexameral  incompletion  in  the  number  of  septa  mak- 
ing up  a  corallite  affects  both  the  entosepta  and  the  exosepta,  that  is,  both 
the  penultimate  and  the  last  cycles.     If  any  septa  are  wanting  to  complete 


I08  SIDERASTREA    RADIANS. 

the  hexameral  multiple  of  the  last  cycle  of  entosepta,  the  same  number 
will  be  lacking  from  the  exosepta.  The  third  complete  cycle,  as  understood 
by  Milne-Bdwards  and  Verrill,  is  really  made  up  of  both  tertiary  entosepta 
and  of  tertiary  exosepta.  The  two  kinds  of  septa  are  of  very  different  value 
in  their  development  and  relations  to  the  polyp,  and,  as  a  matter  of  fact, 
will  be  scarcely  of  the  same  thickness  and  radial  length  to  justify  their 
being  regarded  as  a  cycle. 

The  cyclic  formula,  as  above  understood,  may  be  written  6,  6,  12,  x, 
where  x  will  represent  any  number  from  i  to  24.  Formulated  in  this  way 
the  number  12  conveys  the  impression  that  the  third  cycle  is  really  com- 
pleted, and  that  all  the  additions  made  will  belong  to  the  next  or  fourth 
cycle,  whereas  they  will  belong  to  both  the  third  and  fourth  cycles.  Beyond 
the  two  first  cycles  the  septa  do  not  arise  a  cycle  at  a  time,  but  the  penulti- 
mate and  last  cycles  are  formed  concurrently,  or  almost  so.  Incomplete 
cyclic  hexamerism,  as  met  with  in  S.  radians,  is  an  intermediate  condition 
in  the  establishment  of  two  adult  hexameral  cycles,  not  of  one  alone,  and 
attention  should  be  drawn  to  this  in  the  septal  formula. 

According  to  the  relationships  now  established  the  morphological  septal 
formula  for  S.  radians  should  be  written  6,  6,  x,  6  +  6  +  x.  In  this 
formula,  6,  6,  x  will  represent  the  number  of  septa  in  the  two  completed 
entocycles,  x  being  the  number  in  the  last  entoseptal  or  penultimate  cycle 
which  does  not  yet  complete  the  hexameral  sequence  ;  while  6  -|-  6  -|-  x  will 
represent  the  total  number  of  exosepta,  x  being  the  same  number  as  before ; 
some  of  the  exosepta  will  be  tertiaries  and  some  will  be  quaternaries,  the 
number  of  the  latter  being  always  double  the  number  of  tertiary  entosepta. 
The  formula  for  a  corallite  having  36  septa  would,  according  to  the  ordinary 
cyclic  formula,  be  written  6,  6,  12,  12,  whereas,  considered  as  entosepta  and 
exosepta,  the  formula  will  be  6,  6,  6,  18,  the  three  first  numerals  indicating 
the  entosepta  and  the  last  the  exosepta  ;  the  usual  cyclic  formula  of  a  corallite 
with  40  septa  would  be  6,  6, 12, 16,  and  the  morphological  formula  6,  6,  8,  20. 
In  the  first  case  12  of  the  exosepta  will  be  quaternaries  and  6  will  be  ter- 
tiaries ;  in  the  second  16  will  be  quaternaries  and  4  tertiaries. 

Where  the  relationships  of  the  septa  to  the  mesenteries  are  clearly  known 
the  morphological  formula  will  more  nearly  express  the  real  value  of  the  septa 
than  the  ordinary  cyclic  formula  ;  the  latter  has  little  significance  unless  the 
hexameral  sequence  is  fully  completed.  One  can  not  say  that  a  cycle  is 
really  complete  unless  its  constituents  all  have  the  same  morphological  value, 
which  is  not  the  case  where  some  are  entosepta  and  some  are  exosepta. 

The  bilaterality  of  the  polyp  during  development  may  be  looked  upon 
as  associated  in  turn  with  each  cycle  individually.     Any  cycle  tends  to  attain 


POSTLARVAL  DEVELOPMENT.  I09 

its  radial  plan  before  the  next  cycle  commences  to  form,  when  the  additions 
take  place  in  such  a  manner  as  to  again  confer  bilaterality  upon  the  polj^p 
as  a  whole.  Thus  the  first  two  cycles  of  septa  become  perfectly  radial 
before  an  additional  cycle  commences,  and  the  growth  of  this  is  then  con- 
tinued in  a  bilateral  manner ;  likewise,  the  new  second  and  third  cycles 
assume  their  radial  stage  before  the  members  of  the  fourth  cycle  make  their 
appearance,  proceeding  from  one  border  to  the  other.  In  like  manner  the 
first-cycle  mesenteries  are  nearly  radial  before  those  of  the  second  cycle  arise 
and  introduce  a  conspicuous  bilateral  symmetry  ;  and  on  these  assuming  the 
radial  plan  the  third-cycle  mesenteries  begin  to  appear,  again  in  a  bilateral 
manner. 

The  successive  dorso-ventral  growth  followed  by  the  constituent  mesen- 
teries and  septa  of  each  cycle  may  also  be  regarded  as  conferring  a  certain 
individuality  upon  the  cycle.  The  difierent  cycles,  arising  independently,  seem 
to  represent  so  many  distinct  recurring  phases  of  growth  in  the  life  of  the 
pol)rp,  not  a  continuous  addition  from  one  aspect  to  the  other,  as  is  usual  in 
permanently  bilateral  animals,  particularly  segmented  forms.  The  members 
of  a  cycle  appear  in  a  dorso-ventral  sequence,  and  may  retain  their  differences 
in  size  for  a  long  time,  but  in  the  end  they  become  equal  and  thereby  confer 
radial  symmetry  upon  the  polyp.  Then  another  cycle  commences  to  form 
in  somewhat  the  same  bilateral  dorso-ventral  succession,  displays  for  a  time 
its  consecutive  origin,  and  afterwards  attains  radiality. 

The  conception  of  recurring  phases  of  growth  in  cyclic  coral  polyps  is 
best  realized  when  comparison  is  made  with  the  mesenterial  increase  char- 
acteristic of  the  Ceriantheae.  Here  the  mesenteries  beyond  the  protocnemes 
always  develop  in  a  regular  bilateral  successive  manner,  from  the  dorsal 
(anterior,  sulcar)  to  the  ventral  (posterior,  asulcar)  aspect,  the  oldest  being 
dorsal  or  anterior  and  the  youngest  ventral  or  posterior,  recalling  more  the 
method  of  growth  of  segmented  animals ;  in  cerianthids  there  is  never  a 
reversal  of  growth  to  the  anterior  end,  followed  by  a  successive  series  to  the 
other,  such  as  occurs  in  ordinary  hexactinians.  Employing  the  term  ''band 
of  proliferation,"  introduced  by  Van  Beneden  in  1897,  we  may  say  there  is 
only  one  median  band  of  proliferation  in  cerianthids,  while  in  hexactinians 
there  are  many  such  bands,  the  number  increasing  with  age — at  first  6,  then 
12,  24,  etc. 

In  the  Zoantheae  also  mesenterial  growth  is  always  in  the  same  succes- 
sion after  the  protocnemic  stage.  The  increase  takes  place  within  the  two 
exocoelic  chambers  on  each  side  of  the  ventral  directives  ;  there  are  only  two 
bands  of  proliferation  or  zones  of  growth.     In  this  case,  however,  the  order 


no  SIDERASTREA    RADIANS. 

followed  by  the  new  mesenteries  differs  from  that  in  hexactinians  and 
cerianthids ;  it  proceeds  from  the  ventral  (posterior,  sulcar)  to  the  dorsal 
(anterior,  asulcar)  aspect  of  the  polyp,  not  from  the  dorsal  to  the  ventral. 

The  bilateral  development  of  the  organs,  from  one  border  of  the  polyp 
to  the  other,  in  ordinary  actinians  and  corals,  would  seem  to  have  no  phylo- 
genetic  significance  beyond  the  group  of  the  coelenterates,  and  as  yet  we 
appear  to  have  no  definite  understanding  as  to  what  even  this  may  be.  The 
approximate  radial  symmetry  of  adult  coelenterates  is  assumed  from  very 
diverse  developmental'  conditions  {cf.  hexactinians,  zoanthids,  cerianthids, 
and  the  tentacles  and  other  cyclic  organs  in  the  Hydromedusae  and  Scypho- 
medusse).  Whatever  may  be  said  in  favor  of  Sedgwick's  well-known  view 
that  the  mesenterial  arrangement  found  in  cerianthids  suggests  the  metamer- 
ism of  higher  animals,  there  is  clearly  no  support  for  such  a  conception  in 
the  development  of  the  organs  in  hexactinians. 

SUMMARY. 

1.  In  Siderastrea  radians  the  six  members  of  the  first  cycle  of  septa 
appear  simultaneously,  shortly  after  fixation  of  the  larva,  situated  within 
the  entocceles  of  the  first  cycle  of  mesenteries. 

2.  Six  members  of  a  second  cycle  are  developed  within  the  primary 
exocceles,  shortly  after  the  primary  cycle  of  entosepta.  They  are  the  tempo- 
rary predecessors  of  a  later  permanent  cycle,  and  arise  either  simultane- 
ously or  in  bilateral  pairs  in  a  dorso-ventral  order.  Later,  each  becomes 
bifurcated  peripherally,  either  by  the  direct  extension  of  the  original  septum 
or  by  the  production  of  separate  fragments  which  subsequently  fuse.  The 
bifurcations  also  appear  in  a  bilateral  dorso-ventral  order. 

3.  The  six  members  of  the  permanent  second  cycle  of  entosepta  arise 
within  the  entocceles  of  the  second-cycle  mesenteries  soon  after  these  make 
their  appearance.  The  two  right  and  left  dorsal  septa  appear  first,  then  the 
two  middle  members,  and,  at  a  much  later  period,  the  two  ventral,  the  series 
thus  exhibiting  a  decided  dorso-ventrality.  In  the  end  they  become  equal, 
and  each  fuses  with  the  central  part  of  the  corresponding  second-cycle  exo- 
septum  previously  developed,  which  now  lose  their  individuality. 

4.  Twelve  members  of  a  temporary  third  cycle  are  situated  within  the 
exocceles  between  the  primary  and  secondary  pairs  of  mesenteries,  and  repre- 
sent the  bifurcated  extensions  of  the  six  primary  exosepta.  The  original 
second-cycle  exosepta  thus  become  the  third  exocoelic  cycle,  their  place 
having  been  taken  by  the  permanent  second  cycle  of  entosepta. 

5.  The  later  development  of  the  septa  in  buds  proves  that  a  new  third 


POSTLARVAL    DEVELOPMENT.  Ill 

cycle  of  12  or  less  septa  arises  on  the  appearance  of  the  pairs  of  third-cycle 
mesenteries,  in  a  similar  manner  to  that  followed  by  the  permanent  second 
cycle.  New  entosepta  appear  within  the  entocoeles  of  the  third-cycle  mesen- 
teries, and  the  bifurcations  of  the  third-cycle  exosepta  become  the  exosepta 
of  the  fourth  cycle. 

6.  The  third-cycle  entosepta,  following  the  mesenteries,  are  developed 
in  a  bilateral  dorso-ventral  order,  but  in  two  series-— first  a  series  within  the 
dorsal  moiety  of  each  sextant,  and  then  a  second  series  within  the  ventral 
part  of  each  sextant. 

7.  Bxosepta  are  present  at  each  cyclic  stage  in  the  growth  of  the  coral- 
lum,  alternating  in  position  and  corresponding  in  number  with  the  sum  of 
the  entosepta.  They  never  become  entosepta,  but  always  constitute  the 
outermost  cycle  of  shorter  septa ;  only  the  entosepta  have  any  ordinal  sig- 
nificance. Until  the  adult  condition  is  reached  the  exosepta  are  the  tempo- 
rary predecessors  of  the  entosepta.  The  developmental  relationships  between 
the  entosepta  and  exosepta  are  closely  comparable  with  those  between  the 
entotentacles  and  exotentacles.  The  law  of  substitution,  first  discovered  by 
Lacaze-Duthiers  for  the  tentacles  of  Hexactiniae,  is  thus  found  to  hold  also 
for  the  septa. 

8.  Where  the  cyclic  hexamerism  of  a  corallite  is  incomplete  the  ordinary 
cyclic  formula  does  not  express  the  true  relationship  of  the  septa.  The  ento- 
septa and  exosepta  vary  in  the  same  degree,  so  that  the  morphological 
septal  formula  for  a  corallite  with  three  entoseptal  cycles  and  one  exoseptal 
cycle  is  6,  6,  X,  6  +  6  +  x,  where  x  may  be  any  number  from  i  to  12. 

9.  The  cycles  of  septa  and  mesenteries  represent  so  many  distinct  recur- 
ring phases  of  growth  all  around  the  polyp,  not  a  continuous  increase  from 
one  extremity  to  the  other  as  in  metameric  animals.  With  the  exception  of 
the  first  the  members  of  each  cycle  follow  a  dorso-ventral  succession,  display 
a  bilateral  symmetry  for  some  time,  and  ultimately  assume  an  approximate 
radial  plan.     The  succession  for  the  third  cycle  of  entosepta  is  twofold. 

BASAL  PLATE. 

G.  von  Koch,  in  the  course  of  his  embryological  studies  of  corals,  found 
a  deposit  of  calcareous  matter  to  take  place  between  the  ectoderm  of  the  base 
and  the  surface  of  attachment  of  the  polyps.  It  is  the  first  part  of  the  skele- 
ton to  be  formed  by  the  activity  of  the  calicoblasts,  and  from  its  position  is 
known  in  coral  literature  as  the  basal  plate.  It  is  present  in  the  photographic 
reproductions  on  plates  4  and  5,  and  its  relationship  to  the  polyp,  at  a  rather 
late  stage,  is  shown  on  plate  9,  fig.  53. 


112  SIDERASTREA    RADIANS. 

In  Astroides  calyailaris  Von  Koch  (1882)  found  the  basal  plate  to  take 
the  form  of  a  thin  circular  disc,  composed  of  spheroidal  or  elliptical  crystal- 
line bodies,  from  0.005  t<^  0.008  mm.  in  transverse  section.  At  first  the 
calcareous  elements  were  arranged  in  an  interrupted  manner,  but  later  the 
openings  were  filled  by  further  deposit  of  skeletal  matter,  and  the  plate  as  a 
whole  became  thickened.  In  Caryophyllia  cyathus  (1897)  the  first  rudiments 
of  the  basal  plate  consisted  of  a  small  central  deposit  surrounded  by  six  thin, 
nearly  triangular  plates,  interseptal  in  position.  For  a  time  these  were  dis- 
tinct from  one  another,  but  later  united  with  the  central  circular  patch,  and, 
by  further  additions,  became  joined  along  their  edges,  thus  constituting  a 
complete  flattened  disc  or  plate.  The  presence  of  a  basal  plate  has  since 
been  recognized  in  many  forms  of  corals,  and  it  is  extremely  doubtful 
whether  it  is  really  wanting  in  any  species. 

For  a  long  time  the  developing  polyps  of  6*.  radians  gave  no  external 
indication  of  any  skeletal  formation  which  could  be  regarded  as  the 
basal  plate.  The  first  evidences  of  the  corallum  were  the  six  entoccelic 
septa,  which  appeared  to  rise  directly  from  the  surface  of  attachment. 
Under  transmitted  light  they  stood  out  as  nearly  opaque  objects,  while  the 
interseptal  spaces  at  this  and  later  stages  were  quite  clear  and  apparently 
devoid  of  any  calcareous  crystals.  For  nearly  two  months  the  laboratory 
notes  contained  the  assertion  that  no  basal  plate  was  developed  in  Siderastrea^ 
for,  excepting  the  presence  of  the  septa  and  epitheca,  there  was  no  interrup- 
tion in  the  ordinary  light  passing  through  the  polyp.  When,  however,  the 
living  or  preserved  polyps  were  examined  by  means  of  polarized  light,  the 
bright  colors  of  the  basal  region  revealed  the  presence  of  crystalline  matter. 
This  is  well  seen  in  the  photograph  reproduced  on  plate  4,  fig.  22,  taken 
with  an  exposure  of  twenty  minutes  to  polarized  light.  Between  crossed 
nicols  the  field  was  black  except  in  the  region  of  the  polyp ;  the  septa  also 
appeared  black  owing  to  their  thickness  and  the  irregular  disposition  of  their 
crystalline  constituents,  while  all  the  interseptal  areas  were  brightly  and 
variously  colored. 

It  is  obvious,  therefore,  that  a  basal  plate  was  already  developed  between 
the  polyp  and  its  surface  of  attachment,  but  was  too  thin  to  offer  any  appre- 
ciable obstruction  to  the  transmission  of  ordinary  light.  The  plate  must  have 
begun  to  form  a  day  or  two  after  fixation  of  the  larva,  for  the  septal  upgrowths 
were  observed  on  the  third  and  fourth  days,  and  the  deposit  of  basal  skeletal 
matter  probably  preceded  them.  As  stated  below,  the  plate  does  not  undergo 
much  thickness,  even  in  later  stages  ;  hence,  at  no  time  would  it  offer  much 
interference  to  the  passage  of  light 


POSTLARVAL  DEVELOPMENT.  II3 

When  any  of  the  young  polyps,  two  or  three  weeks  old,  were  macerated 
later,  it  was  found  that  a  thin  continuous  plate  remained  adherent  to  the 
incrusted  surface ;  but  none  of  the  earliest  stages  in  its  formation  were 
obtained,  showing  whether  it  originated  as  a  continuous  layer  or  in  sepa- 
rate parts.  All  the  poly  pal  tissues  have  been  macerated  from  the  young 
coralla  represented  on  plate  4,  figs.  19  and  20 ;  they  now  show  a  well-developed 
basal  plate,  slightly  upturned  at  the  periphery^^and  bearing  the  septa  on  the 
upper  surface. 

A  portion  of  the  basal  plate  of  a  macerated  corallum  of  two  weeks,  magni- 
fied about  300  times,  is  represented  in  surface  view  on  plate  11,  fig.  69.  At  the 
upper  boundary  of  the  figure  the  epitheca  also  is  shown  in  section,  and  upon 
the  plate  are  the  first  thickenings  which  will  form  the  septa.  The  entire 
surface  of  the  plate  exhibits  a  number  of  very  thin,  flat  scales,  roughly  polyg- 
onal in  form.  Sometimes  they  appear  as  if  joined  edge  to  edge  like  the 
cells  of  an  endothelium,  or  at  other  places  as  if  overlapping.  The  average 
diameter  of  the  individual  scales  is  about  0.03  mm.  In  most  of  them  a  distinct 
fibrous  structure  can  be  recognized,  the  fibro-crystals  either  lying  parallel 
or,  less  often,  presenting  a  radiating  appearance.  There  is,  however,  no 
suggestion  of  the  fibers  being  arranged  around  a  center  of  calcification,  as  in 
the  trabeculae  of  the  septa. 

According  to  Miss  Ogilvie  (1897,  pp.  114-117)  similar  scale-like  elements 
are  present  on  the  surface  of  the  corallum  of  corals  generally,  and  she  has 
succeeded  in  isolating  them  from  the  almost  transparent  dissepiments  of 
Galaxea.  Their  breadth  in  this  genus  varies  on  an  average  from  r  ,01  to 
0.015  mm.,  while  their  height  is  about  0.003  ^^^-  A  number  of  fibers  are 
present  in  each,  sometimes  in  the  form  of  divergent  groups,  but  often  lying 
loosely  side  by  side.  It  was  from  a  study  of  these  that  Miss  Ogilvie  came 
to  the  conclusion  that  each  isolated  skeletal  element  represented  a  calcified 
calicoblast  cell,  thus  returning  to  the  old  view  of  von  Heider  as  to  the  origin 
of  the  calcareous  skeleton  of  corals  in  contrast  with  the  more  accurate  results 
of  von  Koch,  Bourne,  and  Fowler,  which  show  that  the  calcareous  matter  is 
secreted  wholly  external  to  the  ectodermal  cells.  The  structure  of  the  calico- 
blast  layer  in  the  polyps  of  Siderastrea  is  also  very  conclusive  as  to  the 
ectoplastic  origin  of  the  calcareous  fibers.  Everywhere  it  has  been  found  to 
be  a  simple  layer,  never  many  cells  deep,  as  would  be  the  case  were  the 
calicoblasts  themselves  calcified  and  shed  from  time  to  time  to  build  up  the 
skeleton. 

From  the  relationships  of  the  basal  plate  to  the  polyp,  increase  in  its  thick- 
ness can  obviously  take  place  only  on  the^upper  surface.     In  addition  to  the 


114  SIDERASTREA    RADIANS. 

septal  rudiments,  small  projections  are  present  liere  and  there  on  this  surface, 
especially  towards  tlie  middle  of  the  plate,  and  some  continue  growing  until 
they  become  raised  much  above  the  general  level  of  the  plate  and  constitute 
columellar  spines.  In  the  basal  plate  we  have  the  skeleton  in  its  earliest 
and  simplest  condition  as  a  flat  deposit,  and  the  septal  and  columellar  forma- 
tions are  vertical  upgrowths  from  it.  The  latter  represent  areas  at  which 
the  calcareous  matter  is  laid  down  with  greater  rapidity  by  the  activity  of  the 
calicoblasts.  The  basal  plate,  however,  is  covered  by  polypal  tissues  on  only 
its  upper  surface,  while  the  upgrowths  from  it — septa  and  columella — are 
covered  on  both  sides ;  hence  the  difference  in  their  microscopic  structure 
shown  on  plate  ii,  fig.  70.  There  are  no  axial  centers  of  calcification  in 
the  elements  of  the  basal  plate,  while  such  are  very  distinct  in  the  septa. 

When  the  young  corallum  is  detached  from  a  smooth  surface,  the  lower 
surface  of  the  basal  plate  is  also  smooth  and  even,  but  when  growing  over  a 
rough  surface  it  adapts  itself  to  the  irregularities. 

The  basal  plate  increased  considerably  in  diameter  from  the  time  of  its 
first  formation,  keeping  pace  with  the  general  growth  of  the  corallum.  In 
the  early  stage  on  plate  4,  fig.  19,  it  is  only  1.3  mm.  across,  while  in  the 
corallum  of  plate  5,  fig.  28,  it  is  2  mm.  in  diameter. 

As  shown  in  the  next  section  an  epitheca  begins  to  form  at  the  edge  of 
the  basal  plate  whenever  growth  ceases,  while  when  lateral  growth  is  con- 
tinued there  is  little  or  no  possibility  of  any  upturned  marginal  deposit. 

Apparently  the  original  basal  plate  of  iS".  radians  never  becomes  much 
thickened.  Some  of  the  oldest  coralla  reared  rather  suggest  that  the  central 
interseptal  portions  may  be  resorbed,  or,  at  any  rate,  remain  extremely  deli- 
cate. When  coralla  of  four  months  were  macerated  and  separated  from  their 
surface  of  attachment  it  was  found  that  interseptally  the  basal  deposit  had 
disappeared  from  the  central  regions,  though  retained  towards  the  periphery 
and  along  the  septa.  It  may  be  that  on  account  of  its  thinness  interseptally 
it  had  broken  away  in  the  process  of  maceration,  while  it  was  supported 
septally  and  peripherally. 

In  colonial  corals  the  basal  plate  is  generally  represented  only  in  the 
corallum  of  the  primary  larval  polyp,  the  later  bud  polyps  not  admitting  of 
its  formation.  But  wherever  in  growing  colonies  of  S.  radians  the  marginal 
corallites  extend  beyond  the  incrusted  object  a  thin  parchment-like  deposit 
is  found  basally.  It  constitutes  the  basal  skeletal  support  of  the  young  bud 
polyps,  in  the  same  manner  as  the  basal  plate  of  larval  polyps,  or  as  the  dis- 
sepiments at  a  later  stage. 


POSTLARVAL    DEVELOPMENT.  II5 

EPITHECA. 

Along  with  the  first  formation  of  the  radiating  septal  upgrowths 
appeared  a  narrow,  peripheral  calcareous  ring,  somewhat  less  opaque  than 
the  septa,  and  for  a  long  time  wholly  unconnected  with  them  (plate  2). 
The  most  careful  examination  of  the  living  polyps  proved  that  the  annulus 
was  altogether  external  to  the  soft  tissues ;  while  in  preserved  and  cleared 
specimens  the  polypal  wall  was  found  to  pass  witliin  the  inner  border  of  the 
skeletal  deposit,  not  to  be  folded  over  it.  The  formation  is  undoubtedly  to 
be  regarded  as  an  epitheca,  according  to  the  definitions  of  this  structure 
given  below. 

The  epitheca  increased  in  height  along  with  the  growth  of  the  polyp, 
at  the  same  time  often  narrowing  a  little  transversely.  It  remained 
throughout  uncovered  by  any  polypal  tissues  on  its  outer  surface,  though 
lined  by  the  polypal  wall  on  its  inner  surface  (plate  9,  fig.  53).  Where 
most  fully  developed  its  outer  surface  exhibited  distinct  incomplete  annula- 
tions,  wrinklings,  or  accretion  lines,  as  if  representing  separate  intervals  in 
the  deposition  of  calcareous  matter  (plate  4).  By  the  time  the  polyps  were 
six  or  seven  weeks  old  the  lower  region  became  discolored  by  the  adherence 
of  foreign  matter,  such  as  filamentous  algae  and  diatoms,  and  only  the  actual 
margin  was  fresh  and  white.  Opposite  the  septa  the  epithecal  margin  was 
sometimes  a  little  indented,  but  otherwise  was  of  the  same  height  all  round, 
somewhat  exceeding  that  of  the  septa.  Whenever,  owing  to  uncongenial 
conditions,  the  polyps  shrunk  from  their  former  size,  the  peripheral  deposit 
remained  behind,  distinct  and  complete ;  the  growth  of  a  second  annulus 
then  took  place  at  the  new  margin  of  the  narrowed  polyp,  in  such  a  way  that 
the  new  ring  was  wholly  within  the  old  (plate  5,  figs.  25-27). 

Later,  as  the  septa  increased  in  radial  length,  some  of  them  came  in 
contact  with  or  were  actually  fused  with  the  epithecal  deposit,  but  the  micro- 
scopic structure  of  the  two  remained  distinct.  When  the  coralla  were  freed 
from  their  surface  of  attachment  the  epitheca  was  seen  to  be  a  direct  upward 
continuation  of  the  edge  of  the  basal  plate,  and,  like  it,  shows  no  centers  of 
calcification.  It  is  made  up  of  circular  lamellae,  the  fibro-crystals  of  which 
are  arranged  at  right  angles  to  the  polypal  wall  (plate  11,  figs.  69,  70) ;  the 
epithecal  deposit,  in  fact,  corresponds  to  but  one-half  of  a  septum. 

The  epitheca  was  found  to  vary  greatly  in  the  extent  of  its  development 
in  the  various  polyps.  In  some  of  the  most  forwardly  developed  specimens 
it  was  practically  absent  or  represented  only  by  a  thickened  marginal  ring 
(plate  4,  figs.   19,  20) ;    in  others  it  appeared  as  a  very  distinct  parapet, 


Il6  SIDERASTREA    RADIANS. 

elevated  beyond  the  level  of  the  septa  (plate  5).  Where  two  or  more  polyps 
developed  contiguous  to  each  other  a  common  epitheca  was  formed  along  the 
line  of  contact  (plate  5,  fig.  30),  but  each  became  distinct  later.  In  one 
instance  two  polyps,  along  with  their  septa,  were  embraced  in  a  common 
epitheca. 

This  surprising  variation  in  the  amount  of  epithecal  deposit  appears  to 
be  determined  by  the  rate  of  growth  of  the  polyp.  As  already  noticed,  the 
polyps  varied  greatly  in  this  respect,  and  the  epitheca  was  best  developed  in 
individuals  which  increased  but  little  in  size.  The  transverse  narrowing  in 
the  later  stages  may  be  taken  to  indicate  that  the  polyps  had  become  some- 
what less  in  their  basal  diameter.  Where  a  polyp  was  growing  rapidly, 
enlarging  the  diameter  of  its  basal  disc  and  extending  the  septa  peripherally, 
it  is  manifest  that  an  epitheca  could  not  be  formed,  or,  if  formed,  would  need 
to  be  resorbed.  In  the  two  coralla  represented  on  plate  5,  figs.  28,  29,  there 
is  practically  no  epitheca,  but  the  basal  plate  is  much  thicker  in  its  peripheral 
half  than  in  the  middle.  Presumably,  the  polyp  did  not  rest  long  enough  at 
any  one  stage  to  permit  of  the  secretion  of  an  epitheca.  Until  maturity, 
therefore,  it  is  possible  that  individual  corallites  of  the  same  species  of 
coral  may  be  provided  with  or  be  destitute  of  an  epitheca,  according  to  the 
slow  or  rapid  rate  of  growth  of  the  polyp ;  the  morphological  value  of  the 
structure  becomes  somewhat  lessened  when  its  formation  is  shown  to  be 
dependent  to  such  an  extent  upon  physiological  conditions. 

The  peripheral  skeletal  formation  in  Siderastrea  is  of  interest  in  con- 
nection with  the  much  discussed  question  as  to  the  nature  of  the  thecal  and 
epithecal  wall  of  corals.  In  some  respects  the  structure  recalls  that  described 
as  theca  (Mauer)  by  von  Koch  (1897)  in  his  paper  on  the  development  of  the 
skeleton  of  Caryophyllia  cyathus^  but  a  comparison  at  once  establishes  their 
different  values.  In  both  species  the  structure  in  question  arises  as  an  inde- 
pendent peripheral  part  of  the  corallum,  and  narrows  from  below  upwards. 
But  in  the  species  investigated  by  von  Koch  the  annulus  is  inclosed  on  both 
its  inner  and  outer  sides  by  an  upgrowth  of  the  basal  wall,  while  in  Sider- 
astrea it  is  external  from  the  beginning,  only  covered  by  the  poly  pal  tissues 
on  its  inner  wall  and  growing  margin  (plate  9,  fig.  53).  Again,  in  the  first 
the  deposit  early  unites  the  edges  of  the  septa,  but  in  the  other  it  remains 
entirely  free  from  septal  connection  for  a  long  time,  and  then  only  joins  their 
edges  as  an  independent  external  covering.  The  different  character  of  the 
theca  (pseudotheca  and  true  theca)  in  various  Madreporaria  has  already  been 
alluded  to  (p.  45),  and  the  original  peripheral  structure  arising  in  Caryophyllia 
would  certainly  belong  to  the  "  true  theca "  type.     Miss  Ogilvie  (1897,  p. 


POSTLARVAL  DEVELOPMENT.  II7 

159)  separates  tlie  theca  from  the  epitheca,  as  follows  :  "  The  essential  differ- 
ence which  may  be  said  to  distinguish  **  theca  "  from  "  epitheca  "  is  that  the 
theca,  or  wall,  must  be  structurally  associated  with  the  peripheral  ends  of 
septa,  whereas  the  epitheca  is  in  no  structural  connection  with  the  septa,  but 
is  a  continuous  concentric  deposit  exterior  to  all  the  other  skeletal  structures 
of  a  calyx."  And  again  (p.  248) :  "  The  epitheca  is  an  external  basal  struc- 
ture, laid  down  at  the  angle  of  the  aboral  wall,  where  it  bends  towards  the  oral 
or  peristomal  region  of  the  polyp  (figs.  22,  36).  It  is  the  continuation  upwards 
or  outwards  of  the  embryonic  *  basal  plate,'  and  may  be  well-developed  or 
remain  rudimentary."  G.  von  Koch  (1896,  p.  254)  describes  the  structure 
in  much  the  same  terms. 

These  two  definitions  serve  to  distinguish  clearly  between  the  periph- 
eral annulus  found  in  Caryophyllia  cyathus  and  that  in  S.  radians.  In 
the  one  case  it  is  structurally  associated  with  the  outer  ends  of  the  septa, 
while  in  the  other  it  is  quite  independent  of  these ;  in  one  it  is  developed 
within  a  special  invagination,  "  thecal  refoulement,"  of  the  skeletotrophic 
wall  which  lines  both  its  inner  and  outer  surfaces,  in  the  other  it  is  wholly 
external  to  the  polyp,  covered  on  one  side  only  by  the  skeletogenic  tissues, 
not  by  an  upfolding.  In  Caryophyllia  von  Koch  found  no  trace  of  an 
epitheca  in  addition  to  the  theca,  while  in  Astroides  and  now  in  the  early 
corallum  of  Siderastrea  there  is  found  to  be  an  epitheca  but  no  true  theca. 
Whatever  calicinal  wall  is  found  in  the  mature  corallum  of  these  two  genera 
is  a  later  structure  formed  by  the  coalescence  of  the  outer  edges  of  the  septa 
(pseudotheca). 

By  Ogilvie  (p.  248)  and  Vaughan  (p.  48)  the  epitheca  is  regarded  as  a 
primitive  structure  in  Madreporaria,  while  the  thecal  structures  are  of 
secondary  origin.  The  latter  writes :  "  The  oldest  type  is  where  the  ends 
of  the  septa  did  not  fuse  distally,  but  simply  had  their  outer  ends  bound 
together  by  an  epithecal  covering."  This  clearly  describes  the  early  epithe- 
cate  corallum  of  Siderastrea.  In  this  respect,  therefore,  the  genus  must  be 
considered  as  representing  an  older  type  than  the  truly  thecate  corallum  of 
such  a  form  as  Caryophyllia.  Its  very  rudimentary  condition  in  some  of 
the  young  polyps  and  its  almost  complete  absence  from  the  adult  colony 
would  seem  to  prove  that  in  this  genus  it  is  an  embryonic  structure  of 
diminishing  importance. 

COLUMELLA. 

For  a  time  the  central  part  of  the  basal  plate  was  free  from  any  calcare- 
ous deposit  which  could  be  regarded  as  a  columella  (plate  4),  but,  as  the  septa 
increased  in  size,  spinous  upgrowths  began  to  form  near  their  inner  extremity, 


Il8  SIDERASTREA    RADIANS. 

extending  as  far  as  tlie  center  of  the  calice  (plate  5).  Some  of  these  upgrowths 
appeared  as  separate  projections  from  the  basal  plate,  as  if  produced  by  special 
invaginations  of  the  basal  wall  of  the  polyp — the  refoulement  columellaire 
of  Delage  &  Herouard  (1901,  p.  558) ;  others  seemed  to  be  direct  continua- 
tions of  the  septa,  not  requiring  a  separate  upfolding  of  the  basal  disc.  The 
former  undoubtedly  represent  a  true  independent  columella,  while  the  latter 
by  their  union  might  give  rise  to  a  so-called  pseudocolumella.  At  no  time, 
however,  could  any  sharp  distinction  be  drawn  between  the  septal  and  the 
independent  upgrowths,  except  as  regards  their  position.  In  the  coralla  rep- 
resented on  plate  4,  figs.  23  and  24,  two  or  three  of  the  middle  granules  are 
obviously  distinct  basal  formations,  while  exactly  similar  spinous  projections 
occur  at  the  end  of  some  of  the  septa.  The  middle  of  the  corallum  on  plate  5, 
fig.  28,  is  also  occupied  by  distinct  spinous  upgrowths,  both  basal  and  septal  in 
character. 

A  further  stage  in  the  columellar  growth  is  represented  in  the  coralla  of 
figs.  28  and  29.  Here  the  intervals  between  the  spines  are  becoming  partly 
occupied  by  the  deposition  of  secondary  calcareous  matter,  so  that  their  act- 
ual origin  is  obscured.  In  the  mature  corallum  it  was  frequently  found 
(P-  53)  that  the  columella  is  minutely  spinous  or  papillose  as  seen  from  the 
surface,  and  that  in  sections  for  some  distance  below  it  remains  spongy, 
becoming  compact  in  the  deeper  regions  by  the  later  deposition  of  calcareous 
matter.  Further,  where  the  coralla  are  very  strongly  calcified  the  spinous 
character  of  the  columella  disappears  even  superficially,  the  interspaces 
being  altogether  occupied  by  the  secondary  deposit,  which  keeps  pace  with 
the  septal  and  columellar  spinous  formations.  These  differences  in  the 
mature  calice  thus  coincide  closely  with  those  represented  in  the  larval  cor- 
alla of  figs.  23  and  29. 

Both  from  its  origin  and  mature  characters  the  columella  of  S.  radians 
is  therefore  fiDrmed  from  three  independent  sources:  (i)  Separate  basal 
upgrowths;  (2)  septal  spines,  continuous  with  the  central  ends  of  the  septa ; 
(3)  a  secondary  deposit  filling  up  the  interstices  between  i  and  2.  Histolog- 
ically the  trabeculae  of  all  three  are  alike  (plate  10,  fig.  65). 

The  columella  of  Siderastrea  is  thus  a  "  true  "  columella,  to  be  distin- 
guished from  a  "  false  "  or  "  pseudocolumella  "  where  there  is  no  direct  basal 
upgrowth,  but  the  entire  structure  is  formed  from  the  inner  septal  edges.  In 
its  development  it  agrees  most  closely  with  the  account  which  von  Koch 
(1897,  p.  769)  gives  of  the  formation  of  the  columella  in  Caryophyllia. 


POSTLARVAL    DEVEI^OPMENT.  II9 


ANATOMY  AND  HISTOLOGY  OF  LARVA  AND  YOUNG  POLYP. 

A  number  of  free-swimming  larvae  were  preserved  in  corrosive  acetic, 
and  later  studied  by  means  of  transverse  and  longitudinal  sections,  when  all 
were  found  to  be  at  about  tlie  same  stage  of  development.  Only  the  impor- 
tant features  in  which  they  differ  from  mature  polyps  will  be  here  noticed. 

The  larval  ectoderm  is  somewhat  broader  Ihan  the  same  layer  in  the 
adult  polyp.  In  sections  it  measures  about  0.04  mm.  across  while  that  of  the 
adult  is  0.03  mm.  The  outer  surface  is  strongly  and  uniformly  ciliated,  the 
enlarged  base  of  each  cilium  being  well  defined.  Numerous  clear  and  gran- 
ular gland  cells  occur,  and  towards  the  margin  a  zone  of  small  nematocysts 
0.015  mm.  in  length.  The  layer  further  differs  from  that  of  the  adult  in 
containing  a  few  Zooxanthellse,  mainly  restricted  to  the  oral  extremity. 
These  have  been  already  noticed  among  the  external  characters  as  giving  a 
brownish  color  to  the  oral  pole  of  the  larva,  and  are  apparently  liberated 
from  time  to  time. 

The  larval  ectoderm  is  broader  at  the  base,  where  it  measures  0.06  mm. 
across,  and,  in  addition,  has  undergone  certain  histological  modifications. 
The  cells  as  a  whole  seem  more  compactly  arranged,  gland  cells  are  less 
numerous,  and  a  nerve  layer  is  present ;  but  nematocysts  do  not  seem  more 
numerous  than  elsewhere.  The  whole  structure  recalls  the  aboral  sense 
organ  which  has  already  been  found  by  McMurrich,  Appellof  (1900),  and 
myself  (1902,  p.  524),  to  occur  in  certain  actinian  and  coral  larvae,  and  is 
evidentl}^  widely,  though  not  universally,  present  in  these  two  groups.  The 
characteristics  of  the  organ,  however,  are  usually  more  conspicuous  than  in 
the  present  species,  especially  as  regards  the  degree  of  development  of  the 
nervous  elements. 

The  stomodaeal  communication  between  the  exterior  and  the  larval  cavity 
was  already  established  in  all  the  larvae  studied.  In  most  the  lumen  is  circu- 
lar, while  in  others  it  is  slightly  oval.  The  surface  is  more  strongly  ciliated 
than  that  of  the  outer  ectoderm,  and  fewer  gland  cells  occur;  the  two  agree 
in  the  presence  of  Zooxanthellae,  though  these  are  never  found  in  the  ecto- 
derm of  the  adult  stomodaeum. 

The  mesenterial  filaments  on  pairs  i  and  11  on  plate  8,  fig.  52,  stand 
out  conspicuously  from  the  endoderm  on  account  of  the  number  and  deeply- 
staining  character  of  their  nuclei,  and  their  histological  structure  recalls  that 
of  the  stomodaeal  ectoderm.  In  the  larvae,  however,  the  organs  are  not  yet 
rounded  ofif  from  the  mesenterial  endoderm. 


I20  SIDERASTREA    RADIANS. 

In  the  serial  transverse  sections  from  whicli  figs.  51  and  52,  on  plate  8, 
were  taken,  the  stomodaeal  ectoderm  appears  to  be  continued  uninterruptedly 
as  the  mesenterial  filaments  down  the  edge  of  the  two  lateral  pairs  of  com- 
plete mesenteries  on  their  becoming  free;  but  the  ventral  and  dorsal  direct- 
ives (hi,  iv),  which  are  only  attached  to  the  stomodaeal  wall  for  part  of  their 
length,  are  without  such  modified  tissue  along  their  free  edge. 

The  continuity  of  the  stomodseal  ectoderm  with  the  mesenterial  fila- 
ments in  the  various  larvae  of  Siderastrea^  and  the  close  histological  simi- 
larity of  the  two,  seem  at  first  sight  undoubted  evidence  of  the  ectodermal 
origin  of  the  latter,  especially  when  it  is  found  that  filaments  are  not  present 
on  mesenteries  which  do  not  reach  the  stomodasum,  or  do  not  extend  as  far 
as  its  inner  termination.  In  the  larvae  of  many  other  Actiniaria  and  Madre- 
poraria,  however,  it  has  been  found  that  filaments  may  appear  on  mesen- 
teries before  they  reach  the  stomodseum,  and  even  in  complete  mesenteries 
an  interval  of  undifferentiated  endoderm  often  occurs  between  the  ter- 
mination of  the  stomodaeal  ectoderm  and  the  filaments.  From  observa- 
tions on  other  corals  I  consider  that  the  mesenterial  filaments  arise  from  the 
larval  endoderm  independently  of  the  stomodaeal  ectoderm,  but  that  continuity 
is  early  established  in  the  case  of  those  mesenteries  which  unite  with  the 
stomodaeum  (1902,  p.  476). 

The  internal  cavity  of  the  larvae  is  Very  limited  in  extent,  the  endoderm 
nearly  filling  the  whole  chamber.  In  the  youngest  examples  mere  slits 
represent  the  lines  along  which  the  polypal  cavity  will  be  formed  later.  The 
cells  of  the  endoderm  are  much  vacuolated,  and  contain  numerous  Zooxan- 
thellae  scattered  throughout.  The  lining  of  the  mesenteries  and  also  of  the 
intervening  portion  of  the  column  wall  is  not  arranged  as  a  simple  epithelial 
layer  such  as  characterizes  the  adult.  Within  most  of  the  intermesenterial 
spaces  the  endoderm  is  greatly  thickened,  and  in  transverse  sections  stands 
out  as  a  very  distinct  triangular  projection  into  the  ccelomic  cavity,  leaving 
only  a  narrow  slit  between  itself  and  the  mesenterial  lining.  The  vertical 
projections  are  the  "  Vorsepten  "  or  prosepta  of  von  Koch  (1897)  ;  they  are 
the  persistent  parts  of  the  mass  of  endoderm  which  at  an  earlier  stage  occu- 
pies the  whole  interior  of  the  early  larva.  On  plate  8,  fig.  52,  taken  from  a 
rather  late  larva,  the  prosepta  are  still  conspicuous,  and  some  are  associated 
with  rudiments  of  the  mesenteries  ;  the  mesenterial  endoderm  is  still  greatly 
thickened,  and  the  central  cavity  is  beginning  to  enlarge. 

In  an  earlier  paper  on  the  larva  of  the  actinian  Lebrunia  coralligens 
(1899),  ^  tave  shown  that  most  anthozoan  larvae  are  for  some  time  nearly 
solid,  owing  to  the  enormous  development  of  the  endoderm.     There  are,  how- 


POSTLARVAL    DEVELOPMENT.  121 

ever,  very  narrow,  canal-like  slits,  and  from  these  the  adult  gastro-coelomic 
cavity  is  derived  by  the  disintegration  of  the  more  central  endoderm  and  the 
shrinkage  of  that  lining  the  body-wall  and  mesenteries. 

Some  of  the  larvae  of  Siderastrea  sectionized  reveal  a  gastro-coelomic 
cavity  further  developed  than  that  represented  in  figs.  51  and  52.  Below  the 
stomodseal  region  the  parenchymatous  endoderm  has  broken  down,  and  the 
middle  of  the  cavity  is  occupied  by  organic  debris,  in  which  Zooxanthellae, 
cell  walls,  and  granules  of  various  kinds  are  recognizable.  There  is  no 
doubt  that  this  is  the  organic  debris  which  is  extruded  from  time  to  time  by 
the  larvae  soon  after  their  liberation  from  the  parent  polyp  (p.  58).  At  the 
stage  in  the  young  polyp  at  which  the  septa  are  well  advanced  (plate  9,  fig. 
53),  the  endoderm  has  become  a  simple  epithelial  layer  throughout,  resem- 
bling in  all  respects  that  of  the  mature  polyps. 

In  the  earliest  larvae  sectionized  eight  mesenteries  were  already  developed, 
arranged  in  four  bilateral  pairs,  as  shown  diagrammatically  in  fig.  8,  <2,  p.  80. 
The  two  lateral  pairs  are  united  with  the  stomodaeum,  while  the  dorsal  and  ven- 
tral axial  pairs,  representing  the  directives,  are  free,  and  of  the  two  directive 
pairs  the  ventral  are  slightly  larger  than  the  dorsal.  In  larvae  a  day  or  so 
older  the  ventral  directives  have  united  with  the  stomodaeum,  while  the  dorsal 
are  still  free  (plate  8,  fig.  51) ;  also  at  the  stage  with  three  complete  mesenteries 
two  other  bilateral  pairs  of  mesenteries,  the  fifth  and  sixth  in  the  sequence,  have 
made  their  appearance,  arranged  as  on  plate  8,  fig.  52.  Afterwards  the  dor- 
sal directive  mesenteries  unite  with  the  stomodaeum,  and  the  larva  has  reached 
the  Bdwardsian  stage  of  mesenterial  development  presented  at  the  time  of 
fixation  (fig.  8,  a-c^  p.  80). 

There  appears  to  be  no  resting  stage  in  the  appearance  of  the  mesen- 
teries between  the  tetrameral  and  hexameral  condition,  such  as  seems  to  char- 
acterize certain  actinians  {Lebruma),  nor  in  the  successive  union  of  the  first 
four  pairs  with  the  stomodaeum.  As  already  shown,  however,  the  fifth  and 
sixth  pairs  remain  as  microcnemes  for  a  prolonged  period. 

As  represented  on  plate  8,  figs.  51  and  52,  the  mesenterial  mesogloea  is 
extremely  thin.  The  cut  ends  of  very  delicate  muscular  fibrils  can  be  recog- 
nized in  transverse  sections,  and,  according  to  their  disposition  on  one  side  or 
other  of  the  mesogloea,  assist  in  the  determination  of  the  paired  arrangement. 

The  further  development  of  the  mesenteries,  after  fixation  has  taken 
place,  can  be  easily  followed  through  the  transparent  tissues  of  the  living 
polyp,  and  has  been  already  described. 


I«2  SIDERASTREA    RADIANS. 


YOUNG  POLYPS. 


Very  few  of  the  young  polyps  reared  were  available  for  anatomical  and 
histological  study,  owing  to  the  greater  importance  of  the  corallum  at  this 
stage  ;  in  most  cases  the  soft  tissues  were  removed  by  maceration  in  order  to 
secure  the  skeleton.  A  vertical  section  through  a  decalcified  retracted  larval 
polyp,  of  about  two  months,  is  represented  on  plate  9,  fig.  53.  The  section 
of  the  skeleton  has  also  been  added,  the  details  being  taken  from  various 
coralla  of  this  age.  The  section  of  the  polyps  includes  the  oral  aperture,  and 
on  the  right  is  truly  radial,  passing  through  a  mesenterial  space  and  cutting 
a  septal  invagination  obliquely,  while  on  the  left  side  it  passes  obliquely 
through  a  mesentery  and  also  through  a  septal  invagination.  A  third  invag- 
ination is  included  in  the  middle  region  of  the  section  and  probably  represents 
an  early  columellar  upgrowth.  The  upper  wall  on  the  left  has  come  to  rest 
upon  one  of  the  invaginations. 

The  septal  and  columellar  invaginations  are  here  of  the  simplest  charac- 
ter. They  are  merely  continuous  upgrowths  of  the  basal  wall  of  the  polyp, 
and  agree  with  it  histologically.  They  line  both  sides  of  the  calcareous 
upgrowths  from  the  basal  plate,  while  the  latter  in  its  turn  is  laid  down  by 
the  flattened  part  of  the  disc,  increase  in  thickness  taking  place  only  on 
the  upper  side. 

Sections  of  coral  polyps  at  such  an  early  stage  are  the  most  favorable  for 
demonstrating  conclusively  that  the  madreporarian  skeleton  is  laid  down 
wholly  outside  the  polypal  tissues,  and  that  all  the  skeletal  complications  are 
formed  within  extensions  of  the  basal  disc,  the  invaginations  proceeding  pari 
passu  with  the  deposition  of  calcareous  .matter.  This  truth  was  first  estab- 
lished by  von  Koch  by  means  of  sections  of  polyps  of  Astroides^  somewhat 
similar  to  that  of  plate  9,  fig.  53  ;  further,  von  Koch  was  able  to  demonstrate 
the  calcareous  spheroids  of  the  basal  plate  and  septa  in  situ.  His  polyps 
for  this  purpose  were  adherent  to  pieces  of  cork,  so  that  the  polyp  and  its 
attachment  could  be  sectionized  together.  The  polyps  of  Siderastrea  being 
adherent  to  glass  could  be  sectionized  only  after  being  freed  through  decal- 
cification. 

Decalcification  of  the  polyp  from  which  fig.  53  was  taken  was  carried  out 
with  great  care,  but  only  fragments  of  a  very  narrow  skeletogenic  ectoderm 
remained.  No  hints  of  any  mesoglceal  processes  or  desmocytes  occurred, 
but  such  would  scarcely  be  expected  considering  their  rarity  in  the  adult 
polyp  oi  Siderastrea,    The  mesogloea  itself  is  a  very  thin  lamella.    The  basal 


POSTLARVAL    DEVELOPMENT.  1 23 

endoderm  also  has  undergone  a  great  alteration  compared  with  the  same 
layer  in  the  other  regions  of  the  polyp.  It  is  much  thinner,  all  traces  of  cell 
limitations  are  lost,  and  Zooxanthellae  are  wholly  absent,  while  elsewhere  in 
the  endoderm  the  algal  cells  occur  in  some  abundance. 

The  passage  from  the  calicoblastic  ectoderm  to  the  ectoderm  of  the 
column  wall  is  gradual,  and  can  be  studied  on  both  sides  of  the  section.  It 
is  at  this  point  that  the  epitheca  is  laid  down.  Were  there  any  doubt  as  to 
the  epithecal  or  thecal  nature  of  the  peripheral  calcareous  ring  such  sections 
prove  absolutely  that  the  deposit  is  uncovered  by  the  polypal  tissues  on  its 
outer  sides.  Were  it  otherwise  an  invagination  of  the  base  would  occur 
towards  the  periphery,  but  none  of  the  sections  show  any  such  folding. 

At  the  region  of  the  epitheca  there  is  no  sharp  distinction  between  the 
column  wall  and  the  basal  disc.  The  ectoderm  of  the  former  is  a  broad 
columnar  epithelium,  having  many  clear  unicellular  mucous  glands  distrib- 
uted among  the  supporting  cells  ;  the  nuclei  are  restricted  mostly  to  the  inner 
half  of  the  layer.  The  left  half  of  the  disc  includes  the  section  of  a  tentacle, 
represented  only  by  the  knob  which  is  crowded  with  long,  narrow  nematocysts. 
As  in  adult  retracted  polyps,  the  stem  of  the  tentacle  is  not  distinct  from 
the  disc.  Histologically  the  walls  of  the  stomodseal  invagination  closely 
resemble  those  of  the  adult  polyps,  and  the  ectoderm  is  marked  off  from  the 
rest  of  the  body  wall  by  its  numerous  ciliated  supporting  cells. 

On  both  sides  of  the  section  represented  in  fig.  53  the  stomodseal  wall 
terminates  freely,  but  in  sections  which  include  a  mesentery  the  ectoderm  is 
seen  to  be  continuous  with  the  mesenterial  filaments  along  the  free  edge  of 
the  mesentery.  The  two,  stomodaeal  ectoderm  and  mesenterial  filament,  are 
much  alike  histologically,  and  by  their  brightly-staining  character  are  easily 
recognizable  among  the  other  tissues. 


REFERENCES. 

(Only  the  more  frequently  recurring  references  are  here  given ;  the  others  are  inserted  in  the  text.) 

1900.  Appellof,  A. :  "  Studien  Uber  Actinien-Entwicklung."     Bcrgens  Museums  Aarbog.     1900,00.  i. 
1897.     VAN  Bknedkn,  E. :  "  Les  Anthozoaires  de  la  Plankton-Expedition."     Rdsultats  de  la '' Plankton- 
expedition  der  Humboldt-Stiftung."    Vol.  11,  Kiel  et  Leipsic. 

1887.  Bourne,  G.  C.  :  "The  anatomy  of  the  Madreporarian  Coral  Fungia."    Quart.  Journ.  Micr,  Sci., 

vol.  XXVII. 

*893- •  "  O"  the  postembryonic  development  of  Fungia."    Trans.  Roy.  Dublin  Soc,  ser.  xi, 

vol.  V. 
1899. :   "Studies  on  the  structure  and  formation  of  the  calcareous  skeleton  of  the  Antho- 

zoa."     Quart.  Journ.  Micr.  Sci.,  vol.  XLi. 

1901.  Delage,  Y.,  and  Herouard,  E. :  "Trait^  de  Zoologie  Concrete.   Les  Coelent6rds."    Tom.  11,  pt.  2. 

1899.  DuERDEN,  J.  E  :  "  The  Edwardsia-stage  of  the  actinian  Lebrunta,  and  the  formation  of  the  gastro- 

coelomic  cavity."    Journ.  Linn.  Soc,  Zool.,  vol.  xxvii. 

1902. :  "  West  Indian  Madreporarian  Polyps."     Mem.  Nat.  Acad.  Sciences,  vol.  viii,  7th  Mem. 

1895.     Faurot,  L.  :  "  ifetudes  sur  I'anatomie,  I'histologie  et  le  d^veloppement  des  Actinies."    Arch,  de 

Zool.  Exp.  et  Gdn.,  ser.  3,  tom.  iii. 

1888.  Fowler,  G.  H.  :  "The  anatomy  of  the  Madreporaria,  IV."    Quart.  Journ.  Micr.  Sci.,  vol.  xxviii. 

1902.  Gardiner,  J.  S. :  "  South  African  corals  of  the  genus  Ftabellttm,  with  an  account  of  their  anatomy 

and  development."     Marine  Investigations  in  South  Africa,  vol.  11,  Cape  Towrn. 
1882.    VON  Koch,  G.  :  "  Ueber  die  Entwicklung  des  Kalkskeletes  von  Asteroides  calyctilaris  und  dessen 

morphologischer  Bedeutung."    Mitt.  a.  d.  Zool.  Stat,  zu  Neapel,  bd.  iii. 

1896. :  "  Das  Skelett  der  Steinkorallen."     Festschrift  fiir  Carl  Gegenbaur.     Leipzig. 

1897. :  "Entwicklung  von  Caryophyllia  cyathus."     Mitt.  a.  d.  Zool.  Stat,  zu  Neapel,  bd.  xii. 

1872.     Lacaze-Duthiers,  H.  de:    "D^veloppement  des  Coralliaires.     Prem.  Mem.,  Actiniaires  sans 

Polypier."    Arch. <ie  Zool.  Exp.  et  Gdn.,  tom.  i. 
1873. •  " D^veloppement  des  Coralliaires.     Deux.  Mdm.,  Actiniaires  a  Polypier."    Arch.de 

Zool.  Exp.  et  G6n.,  tom.  11. 
1897. :  "  Faune  du  Golfe  du  Lion.  Coralliaires.    Zoanthaires  Sclerodermas."    Arch,  de  Zool. 

Exp.  et  Gdn.,  3  ser.,  tom.  v. 
1897.     Ogilvie,  M.  :  "  Microscopic  and  systematic  study  of  Madreporarian  types  of  corals."    Phil.  Trans., 

vol.  CLXXXVII. 

1900.  Vaughan,  T.  W.  :  "  The  Eocene  and  Lower  Oligocene  Coral  Faunas  of  the  United  States."     U.  S. 

Geol.  Survey,  Monogr.  xxxix. 

1901.  Verrill,  a.  E.  :  "Variations  and  nomenclature  of  Bermudian,  West  Indian  and  Brazilian  Reef 

corals,  with  notes  on  various  Indo-Pacific  corals."    Trans.  Conn.  Acad.  Science,  vol.  xi. 
1888.     Wilson,  H.  V.  :  "On  the  development  oi Manicina  areolata"    Journ.  Morph.,  vol.  11  C1889). 


EXPLANATION  OF  PLATES. 

Plate  i. 

Fig.  I. — Larva  immediately  on  extrusion,  viewed  by  reflected  light.  The  uniform  covering  of  cilia  is 
indicated,  and  also  the  distinction  between  the  outer  ectoderm  and  the  solid  internal  endoderm.  The 
darker  color  of  the  broader  oral  pole  is  due  to  the  presence  of  Zooxanthellae  in  the  ectoderm.  The 
narrower  aboral  pole  is  anterior  in  swimming;  the  mouth  is  not  yet  functional. 

Fig.  2. — Abnormal  larva  with  two  oral  poles  and  one  aboral. 

Fig.  3. — Larva  shortly  after  extrusion.  The  larva  is  now  more  swollen  and  transparent,  so  that  four 
pairs  of  mesenterial  lines  are  seen;  the  mouth  is  also  functional.  Two  pairs  of  mesenteries  reach  the 
stomodseum,  and  two  other  pairs  are  free. 

Fig.  4. — A  second-day  larva  just  before  settling.  Six  pairs  of  mesenteries  are  now  present,  three 
pairs  of  which  reach  the  stomodseum. 

Fig.  5. — Three  larvee  settling  close  together  by  the  narrow  aboral  pole. 

Fig.  6. — A  group  of  seven  third-day  larvae  which  have  settled  so  close  together  that  their  walls  partly 
overlie.  Four  pairs  of  mesenteries  now  reach  the  stomodeeum,  and  on  three  of  the  individuals  the 
rudiments  of  the  six  exocoelic  tentacles  have  appeared. 

FiG.  7. — A  living  polyp  two  or  three  days  after  settling,  viewed  by  transmitted  light.  The  dark 
outermost  rim  represents  the  epitheca;  the  next  lighter  zone  is  the  flat  margin  of  the  polypal  wall  into 
which  the  polypal  cavity  does  not  extend.  The  six  entosepta  are  opaque,  but  in  such  a  view  there  is  no 
evidence  of  the  basal  plate.  The  six  tentacles  are  exocoelic  and  alternate  with  the  septa.  The  internal 
Zooxanthellae  have  accumulated  mainly  along  the  sides  of  the  mesenteries;  the  ectodermal  Zooxanthella 
have  disappeared  except  immediately  around  the  mouth.     Diameter  of  original,  1.5  mm. 

Plate  2. 

Fig.  8. — Living  polyp  with  a  dorso-lateral  pair  of  exosepta  and  a  rudimentary  pair  of  median  exosepta 
in  addition  to  the  six  entosepta. 

Fig.  9  — Polyp  with  six  entosepta  and  six  exosepta,  the  latter  still  revealing  their  dorso-ventral  order 
of  development  by  differences  in  maj^nitude. 

Fig.  10. — A  living  polyp  of  about  the  same  stage  as  fig.  9,  fully  expanded  and  viewed  from  the  side 
as  a  transparency.     Of  the  mesenteries  only  the  insertions  are  represented. 

Fig.  ii. — Polyp  with  twelve  fully  expanded  tentacles,  six  large  outer  exocoelic  and  six  small  inner 
entocoelic.     Fifth  week. 

Fig.  12. — Polyp  showing  the  irregular  manner  in  which  peripheral  additions  are  made  to  the  primary 
septa.    The  ventral  directive  tentacle  is  double.     Diameter  of  original,  1.7  mm. 

Plate  3. 

Fig.  13.— Expanded  disc  showing  the  doubling  of  certain  of  the  entotentacles  and  the  asymmetrical 
position  of  others. 

Fig.  14— Expanded  polyp  viewed  from  the  side  so  as  to  display  the  manner  of  appearance  of  the 
second-cycle  mesenteries  on  the  column  wall. 

Fig.  15. — Expanded  disc  resting  on  the  septa  and  exhibiting  the  relationship  of  the  second-cj'cle  mesen- 
teries to  the  new  entosepta  and  exotentacles.  An  additional  exotentacle  has  appeared  within  the  middle 
sextant  on  each  side,  and  the  dorso-lateral  entotentacles  have  each  a  single  stem  bifurcated  distally. 

Fig.  16. — Expanded  disc  showing  the  manner  of  increase  of  the  tentacles.  An  exotentacle  protrudes 
from  each  of  the  two  exocoeles  in  the  dorso-lateral  and  median  sextants,  but  as  yet  there  is  only  one  from  the 
ventro-lateral  sextants.     The  tentacle  from  each  dorso-lateral  entocoele  is  already  bifurcated  as  in  the  adult. 

Fig.  17. — The  same  polyp  as  in  fig.  16  at  a  later  stage.  On  each  side  a  second-cycle  tentacle  (II)  has 
grown  out  over  the  entocoele  of  the  median  second-cycle  mesenteries,  and  the  four  lateral  first-cycle  ento- 
tentacles (I)  are  bifurcated.     Diameter  of  original,  2  mm. 

Fig.  iS. — A  larval  polyp  of  three  months  in  which  the  disc  and  tentacles  are  indrawn  and  the  disc  is 
almost  covered  by  the  overfolding  column  wall.    The  epitheca  is  shown  around  the  margin. 

126 


EXPLANATION  OF  PLATES.  1 27 

Plate  4. 

Fig.  19. — Photograph  of  early  corallum  with  six  entosepta  and  six  exosepta.  The  basal  plate  (proto- 
theca)  is  fully  formed,  but  somewhat  irregular  in  outline;  it  is  a  little  thicker  and  upturned  towards  the 
margin.  The  six  entosepta  are  nearly  equally  developed,  while  the  alternating  exosepta  vary  much  in 
size;  the  dorso-lateral  pair  are  best  developed  and  peripherally  seem  double,  the  middle  pair  are  a  little 
smaller,  while  the  ventro-lateral  are  yet  rudimentary.     Diameter,  1.6  mm. 

Fig.  20. — A  somewhat  later  corallum.  The  directive  entosepta  are  irregular  in  form  and  differ 
from  one  another,  the  ventral  appearing  as  if  bifurcated  peripherally;  the  two  dorso-lateral  and  right 
middle  exosepta  also  appear  bifwrcated ;  the  ventro-lateral  exosepta  are  still  much  smaller  than  the  other 
exosepta.  —    - 

Fig.  21. — A  young  preserved  polyp  photographed  by  transmitted  light.  In  such  a  view  the  basal 
plate  is  not  apparent  owing  to  its  transparency.  The  septa  are  non-transparent  and  stand  out  as  black 
objects.  Indications  of  the  six  pairs  of  radiating  mesenteries  are  seen  and  also  some  of  the  tentacles,  but 
the  details  around  the  mouth  are  obscured  by  the  mesenterial  filaments  and  stomodaaum.  The  dotted 
surface  of  the  polyp  is  due  to  the  presence  of  internal  Zooxanthellae.  The  septa  are  a  little  further 
developed  than  in  fig.  20,  the  exosepta  still  showing  a  decided  dorso-ventrality.  There  is  a  marked 
difference  in  form  between  the  dorsal  and  ventral  directive  septa.  Diameter,  1.7  mm.  (In  the  process  of 
engraving  the  principal  axis  of  the  polyp  has  been  turned  counter-clockwise  through  an  angle  of  about 
20  degrees  from  the  vertical.) 

Fig.  22. — The  same  polyp  as  in  fig.  2i"photographed  by  an  exposure  of  20  minutes  to  polarized  light 
between  crossed  nicols.  Only  the  calcareous  crystals  transmitted  the  variously  colored  light.  The 
mottled  surface  represents  the  basal  plate  which  is  not  indicated  in  the  previous  figure.  The  crystalline 
matter  of  the  septa  is  so  thick  and  irregularly  arranged  that  no  light  was  transmitted ;  hence  the  structures 
appear  black. 

Fig.  23. — A  typical  corallum  at  a  later  stage,  growth  having  proceeded  upon  the  septal  plan  of  figs.  19 
and  20.  The  polyp  remained  at  this  stage  for  some  time,  and  a  well  developed  wrinkled  epitheca  has 
formed,  its  upper  edge  higher  than  the  septa ;  the  latter  are  fused  peripherally  with  the  epitheca,  but  there 
is  no  theca.  The  directive  septa  are  somewhat  bifurcated  peripherally  and  also  the  dorso-lateral  and 
median  exoseptal  pairs.  The  dorso-lateral  pair  are  almost  fused  centrally  with  the  dorsal  directive,  the 
middle  exosepta  are  fused  with  the  dorso-lateral  entosepta  on  each  side,  and  the  ventro-lateral  exosepta 
with  the  ventro-lateral  entosepta.  The  ventro-lateral  exosepta  are  the  smallest  of  the  whole  series.  Sepa- 
rate columellar  nodules  are  present  in  the  middle.  For  diagrammatic  plan  see  p.  88.  Diameter,  1.8 
mm.  (In  the  process  of  engraving  the  principal  axis  of  the  corallite  has  been  turned  clockxvise  through 
an  angle  of  about  20  degrees  from  the  vertical.) 

Fig.  24. — Another  corallum  at  about  the  same  stage  as  fig.  23,  but  showing  variation  in  detail. 

Plate  5. 

Fig.  25. — A  corallum  in  which  the  epitheca  has  gradually  receded  from  the  margin  towards  the 
middle  of  the  calice  as  the  polyp  shrunk  in  size.  The  older  part  of  the  epithecal  deposit  covers  the 
peripheral  parts  of  the  septa.  The  newest  portion  of  the  epitheca  is  the  inner  annulus.  The  septa  are 
at  about  the  same  stage  as  in  fig.  23. 

Fig.  26. — Corallum  in  which  the  polyp  has  twice  diminished  in  size,  somewhat  suddenly.  Three 
concentric  epithecae  have  been  formed,  the  peripheral  ends  of  the  septa  being  partly  exposed  between  each. 

Fig.  27.— The  septa  are  a  little  further  advanced  than  in  figs.  23-26.  In  the  left  dorso-lateral  sextant 
there  are  three  distinct  septa,  a  large  median  and  two  lateral;  the  median  septum  consists  of  a  larger 
central  (exoseptum)  and  a  small  peripheral  part  (entoseptum).  In  the  right  middle  sextant  are  also  three 
septa,  the  median  of  which  consists  of  a  central  and  a  small  peripheral  part  {cf.  fig.  15). 

Fig.  28.— This  and  the  next  corallum  are  the  two  oldest  reared.  Each  contains  three  complete  cycles 
of  septa,  and  some  of  the  sextants  show  important  stages  in  the  passage  from  the  second  to  the  permanent 
third  cycle.  In  the  polyps  which  formed  the  coralla  the  first  and  second  cycles  of  mesenteries  were  fully 
developed.  The  diagrammatic  representation  of  the  stage  is  given  on  p.  89,  and  emphasizes  the  manner 
of  fusion  of  the  various  septa  with  one  another.  The  directive  septa  are  stouter  than  any  of  the  others. 
The  three  septa  within  the  ventro-lateral  sextants  are  not  so  strongly  developed  as  those  within  the  median 
and  dorso-lateral  sextants,  and  show  very  distinctly  the  twofold  constitution  of  the  median  entoseptum 


128  EXPLANATION  OF  PLATES. 

and  the  independent  character  of  theexoseptum  on  each  side  {cf.  fig.  15).  Synapticula  now  connect  some 
of  the  adjacent  septa.  The  origin  of  part  of  the  columella  from  nodular  upgrowths,  independent  of  the 
inner  septal  edges,  is  very  evident.  The  basal  plate  is  thicker  peripherally  than  centrally,  and  the  epitheca 
is  only  feebly  developed  at  the  margin.     Diameter  of  original,  3  mm. 

Fig.  29. — The  details  presented  are  practically  the  same  as  in  the  previous  figure,  but  the  growth  of 
the  three  septa  within  the  ventro-lateral  sextants  has  reached  the  same  stage  as  in  the  other  sextants. 
The  epitheca  is  better  developed  than  in  fig.  28. 

Fig.  30. — Double  corallum  produced  by  two  ptjlyps  the  larVBB  of  which  settled  close  together.  Each 
has  an  independent  epithecal  formation,  the  part  common  to  both  being  very  irregular.  The  septa  have 
not  preserved  the  regular  hexameral  plan. 

Plate  6. 

Fig.  31. — Three  fully  expanded  adult  polyps,  showing  the  forms  assumed  by  the  polyps  and  the 
arrangement  of  the  bilobed  and  simple  tentacles. 

Fig.  32. — Nearly  mature  polyp  preserved  in  formalin  in  a  partly  expanded  condition;  surface  view. 
The  tentacles  over  the  third-cycle  entosepta  (III)  are  still  simple  and  resemble  the  exotentacles  (X). 

Fig.  33. — An  isolated  interseptal  lamella  from  a  decalcified  polyp.  The  lower  margin  represents  the 
part  of  the  lamella  which  rested  upon  a  dissepiment,  and  about  the  middle  nearly  surrounded  a  synap- 
ticulum.  The  apertures  (syti)  are  the  spaces  formerly  occupied  by  the  synapticula.  The  column  wall, 
disc,  a  bifurcated  tentacle,  stomodjeum,  and  a  mesenterial  filament  are  shown. 

Fig.  34. — Transverse  section  of  a  mature  polj'p,  passing  through  the  stomodaeal  region.  The  figure 
shows  the  arrangement  of  the  mesenteries,  the  septal  invaginations,  and  sj'napticula.  In  this  and  other 
figures  the  spaces  occupied  by  the  skeleton  are  represented  by  the  dotted  areas. 

Fig.  35. — Transverse  section  through  a  portion  of  the  calicinal  ridge  of  two  retracted  polyps,  showing 
the  continuity  between  the  mesenterial  chambers  of  adjacent  polyps,  and  also  the  vertical  free  edge  of  the 
mesenteries  as  they  extend  from  the  column  wall  to  the  skeletotrophic  tissues.  In  the  upper  part  the 
septa  are  exsert,  but  below  they  are  united  with  the  calicinal  wall  {ex.  sept.,  exoseptum;  ent.  sept., 
entoseptum). 

Fig.  36. — Tangential  section  towards  the  periphery  of  a  polyp.  The  column  wall  rests  upon  the  septal 
edges,  and  the  mesenteries  have  a  short  vertical  extent. 

Plate  7. 

Fig.  37. — a,  fully  expanded  entotentacle ;  b,  fully  expanded  exotentacle. 

Fig.  38. — Transverse  section  of  the  same  polyp  as  in  fig.  34,  taken  a  little  below  the  stomodaeal  region. 

Fig.  39. — Transverse  section  of  a  polyp  towards  its  aboral  region.  The  mesenterial  loculi  are  alto- 
gether isolated  at  this  level,  and  appear  further  broken  up  hy  the  synapticula  stretching  from  one  septum 
to  another.  The  mesenteries  have  almost  disappeared  and  the  middle  of  the  section  is  occupied  by  the 
columella.     The  skeletal  ectoderm  and  endoderm  are  both  represented. 

Fig.  40. — Tangential  section  through  the  tentacular  region.  A  single  bilobed  entoccelic  tentacle  is 
almost  resting  upon  a  septal  edge.  In  the  septal  invagination  {sep.  inv.)  the  calicoblast  layer  is  repre- 
sented. 

Fig.  41. — Transverse  section  through  a  mesentery  at  the  level  represented  in  fig.  35.  The  meso- 
gloeal  plaitings  for  the  support  of  the  retractor  muscles  are  simple  in  character,  the  oblique  musculture 
is  feebly  seen,  and  also  the  desmoidal  processes  where  the  mesentery  is  attached  to  the  skeletotrophic 
layer  {sk.  I.). 

Fig.  42. — Transverse  section  through  a  mesenterial  filament  immediately  below  the  stomodseal  region. 

Fig.  43. — Transverse  section  of  a  mesentery  some  distance  below  the  stomodeeal  region,  showing  the 
form  of  the  mesenterial  filament,  and  also  a  single  ovum  ;  the  peripheral  end  of  the  mesentery  is  under- 
going resorption. 

Fig.  44. — a,  small  nematocysts  from  the  knob  of  the  tentacle  and  column  wall ;  b,  elongated  form  of 
nematocysts  from  the  knob  of  the  tentacle  and  mesenterial  filaments ;  c,  large  oval  nematocyst  from  the 
lower  part  of  the  mesenterial  filaments. 


EXPLANATION   OF   PLATES.  1 29 

Plate  8. 

Fig.  45. — Section  through  part  of  a  mesentery  in  union  with  the  skeletotrophic  tissues  where  they 
surround  a  synapticulum.  The  mesenterial  mesogloea  for  some  distance  from  its  termination  is  finely 
striated,  forming  a  desmoidal  process.  The  skeletotrophic  ectoderm  on  the  left  side  shows  faintly  a 
narrow  portion  of  the  homogeneous  skeletal  matrix. 

Fig.  46. — Transverse  section  through  an  isolated  portion  of  the  lining  wall  of  an  interseptal  loculus. 
Each  lateral  extremity  was  terminated  by  a  synapticulum.  The  section  was  stained  in  iron  haematoxylin, 
and  shows  very  distinctly  the  mucous  spaces  in  the  inner  endoderm  and  calicoblast  layer.  The  latter  also 
contains  coarsely  granular  cells  and  nematocysts. 

Fig.  47. — Transverse  section  through  the  skeletotrophic  layers  in  their  uppermost  part,  showing  the 
very  narrow  endoderm  and  calicoblast  layer.  The  layers  are  syncytial  in  character.  Irregular  canal-like 
spaces  are  filled  with  a  deeply-staining  substance.  Small  irregular  particles  are  adherent  to  the  free 
surface  of  the  ectoderm. 

Fig.  48. — Transverse  section  through  a  part  of  the  skeletotrophic  layers  from  the  lower  region.  The 
section  gives  practically  the  same  details  as  fig.  46,  but  is  more  highly  magnified. 

Fig.  49.— Longitudinal  section  through  an  expanded  tentacle.  The  stem  is  simple  in  character,  while 
the  swollen  knob  has  a  nerve  and  a  muscle  layer  at  the  base  and  nematocysts  and  gland  cells  {gr.  g.  c.) 
towards  the  periphery. 

Fig.  50. — Transverse  section  through  a  portion  of  a  mesentery  showing  the  system  of  irregular 
spaces  in  the  endoderm.  The  Zooxantheliae  and  cut  ends  of  the  retractor  muscle  fibers  arranged  along 
mesoglceal  plaitings  are  also  represented. 

Fig.  51. — Transverse  section  through  the  stomodaeal  region  of  a  larva  preserved  shortly  afler  extru. 
sion.  The  six  primary  pairs  of  mesenteries  are  present,  but  the  dorsal  directives  (iv,  iv)  and  pairs  v  and 
VI  are  yet  free  from  the  stomodaaum.  The  gastro-ccelomic  cavity  is  beginning  to  be  established  in  the 
nearly  solid  endoderm;  Zooxanthellse  are  present  in  the  thick  stomodasal  ectoderm.  In  this  and  the 
next  figure  the  histological  details  of  the  endoderm  are  represented,  while  the  ectoderm  is  convention- 
ally depicted. 

Fig.  52. — Transverse  section  through  the  same  larva  a  little  below  the  stomodoeal  region.  Mesen- 
terial filaments  are  forming  at  the  free  end  of  the  first  and  second  pairs  of  mesenteries.  Endodermal 
thickenings  (Prosepten)  are  present  between  the  larger  pairs  of  mesenteries. 

Plate  9. 

Fig.  53.  — Radial  vertical  section  through  a  young  polyp,  showing  its  relationship  to  the  early  coral- 
lum.  In  the  latter  are  shown  the  basal  plate  (A.  //.),  epitheca  («/.),  two  septa,  and  acolumellar  upgrowth 
{col.) 

Figs.  54-60. — Serial  transverse  sections  through  a  portion  of  a  polyp,  showing  the  relationships  of 
the  mesenteries  and  septa  in  the  development  of  a  third-cycle  entoseptum  (d-c)  and  two  fourth-cycle 
exosepta  (a-6,  c-cf).     The  full  description  of  the  series  is  given  on  p.  100. 

Plate  10. 

Fig.  61. — A  free  corallum.     Natural  size. 

Fig.  62. — A  portion  of  the  same  corallum  magnified  about  four  times,  showing  various  characteristics 
of  the  corallites  and  their  relationships  to  one  another. 

Fig.  63. — Upper  part  of  three  septa  as  seen  in  a  vertical  surflice  view.  The  left  and  middle  septa  extend 
across  a  calice,  but  are  separated  in  the  middle  by  the  columella  and  the  broken  central  edges  of  other 
septa  in  union  with  it.  The  right  septum  belongs  to  an  adjacent  calice,  and  is  separated  from  the  middle 
septum  by  a  vertical  ridge  which  represents  the  lineof  fusion  of  an  adjacent  septum  not  in  the  same  radius. 
The  two  or  three  vertical  rows  of  large  elevations  on  each  septum  represent  the  broken  surfaces  of  synap- 
ticula  ;  the  smaller  elevations  are  granules.  The  inner  central  margin  of  the  left  septum  shows  the  denti- 
culations  as  they  unite  with  the  columellar  tangle,  and  near  them  the  fractured  surfaces  indicating  where 
an  adjacent  septum  was  interruptedly  united  by  its  inner  margin. 

Fig.  64. — Ground  surface  of  a  portion  of  a  corallum  showing  the  relationship  of  a  single  calice  to  the 
six  adjacent  calices.  Surface  view.  The  long  axis  of  the  columella  represents  the  principal  axis  of  the 
calice,  the  directive  septa  being  at  opposite  extremities,  and  not  quite  in  the  same  plane. 

Fig.  65. — Portion  of  a  thin  transverse  section  of  a  single  corallite  showing  the  microscopic  structure. 
The  columella,  septa,  and  synapticula  seem  as  if  constituted  of  spheroids  fused  together;  the  spheroid* 
represent  transverse  sections  through  so  many  individual  trabeculaa. 


130  EXPLANATION  OF   PLATES. 

Plate  ti. 

Fig.  66. — Transverse  section  through  a  single  trabecula,  penetrated  hy  filamentous  boring  algse,  and 
showing  the  growth  lamellae  and  their  fibrous  character. 

Fig.  67. — Vertical  section  through  a  part  of  two  adjacent  septa.  The  septa  are  made  up  of  trabeculae 
arranged  in  a  radiating  manner;  they  diverge  from  a  continuous  median  trabecula  which  represents  the 
boundary  between  the  two  adjacent  calices.  The  different  appearances  presented  by  the  various  trabec- 
ulae are  dependent  upon  the  part  included  in  the  section.  Where  the  section  passes  along  the  middle  of 
a  trabecula  the  interrupted  dark  centers  of  calcification  are  seen ;  elsewhere  only  the  diverging  bundles  of 
fibro-crystals  appear.  The  growth  lamellte  are  not  shown  (cf.  fig.  68).  New  trabecule  are  intercalated 
at  intervals.     The  dark  circular  and  oval  patches  are  sections  of  synapticula. 

Fig.  68. — Terminal  portion  of  two  trabeculaa  as  seen  in  longitudinal  radial  section,  that  is,  parallel  to 
the  surface  of  the  septum,  more  highly  magnified  than  in  the  previous  figure.  The  section  passes  along 
the  middle  of  the  trabeculae  so  that  all  the  dark  centers  of  calcification  are  seen.  In  the  corallite  from 
which  the  section  was  taken  the  growth  lamellae  were  very  distinct,  the  boundary  of  each  being  indicated 
by  a  dark  granular  deposit  similar, to  that  at  the  centers  of  calcification.  The  growth  lamellae  are  arranged 
parallel  with  the  toothed  edge,  that  is,  with  the  calicoblast  layer  which  secretes  them,  while  the  fibro- 
crystals  making  up  the  lamellae  are  arranged  at  right  angles.  The  figure  should  be  compared  with  fig. 
66,  which  shows  a  trabecula  in  transverse  section. 

Fig.  69. — Portion  of  basal  plate  and  epithecal  boundary  of  a  very  early  corallum.  The  epitheca  is  seen 
in  section,  and  the  basal  plate  is  sufficiently  thin  to  allow  of  the  passage  of  light.  The  basal  plate  is  made 
up  of  more  or  less  distinct  granules  or  scales  showing  a  fibrous  structure;  the  aggregation  of  darker 
granules  represents  the  first  formation  of  a  septum. 

Fig.  70. — Portion  of  basal  plate,  septa,  and  epitheca  of  a  somewhat  older  eorallum  than  that  of  fig.  69. 
The  septa  and  epitheca  are  ground  down  to  nearly  the  level  of  the  basal  plate.  Centers  of  calcification 
are  not  present  in  the  basal  plate  and  epitheca  but  are  very  prominent  in  the  septa. 


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